Complex Magnetic Ordering in Eu 3 InP 3 : A New Rare Earth Metal Zintl Compound

Eu 3 InP 3 has been prepared as large single crystals with an indium flux reaction. The structure of the new compound is isotypic to Sr 3 InP 3 and crystallizes in the orthorhombic space group Pnma with unit cell dimensions of a ) 12.6517(15) Å, b ) 4.2683(5) Å, and c ) 13.5643(14) Å (Z ) 4, T ) 140 K, R1 ) 0.0404, wR2 ) 0.0971 for all data). The structure consists of one-dimensional chains of corner-shared distorted [InP 2 P 2 / 2 ] 6 - tetrahedra separated by rows of Eu 2 + ions. Two of the three crystallographically distinct europium sites have a short Eu(1) - Eu(2) distance of 3.5954(7) Å, which yields Eu - Eu dimers. The Eu - P bond distances range from 2.974(2) to 3.166(2) Å. The temperature dependence of the conductivity indicates that Eu 3 InP 3 is a small band gap semiconductor. Both magnetization and Eu-151 Mo¨ssbauer spectral measurements indicate that the europium in Eu 3 InP 3 is divalent and that at least two magnetic transitions occur. Magnetization studies reveal magnetic transitions at 14, 10.4, and (cid:24) 5 K. These transitions are also observed in heat capacity studies of Eu 3 InP 3 . The Mo¨ssbauer spectra indicate that the two europium sites are ordered at 12 K and that all three europium sites are ordered at 8 K.


Introduction
Zintl phases are valence-specific compounds formed between electropositive elements, such as alkali metal, alkaline earth metal, and, more recently, rare earth elements, and the main group elements. [1][2][3][4] There are a significant number of Zintl compounds composed of the alkali or alkaline earth metals and the elements of groups 13-16. Although there is less known about the rare earth compounds, there has been a recent flurry of research 5 on them because of their promising thermoelectric properties. [6][7][8][9][10][11][12][13] In addition, superconductivity has been discovered in several Zintl compounds. 5,13-15 Because of their unique magnetoresistive properties, [16][17][18][19][20][21][22] we have been exploring a flux synthesis * Author to whom correspondence should be addressed. E-mail: smkauzlarich@ucdavis.edu. †  technique for growing large crystals of the Ln 14 MnPn 11 compounds, where Ln is Eu or Yb and Pn is P, As, Sb, or Bi. This research effort has led to the discovery of several new compounds with unique magnetic and electronic properties. [23][24][25] Several ternary, europium-containing transition metal pnictides have been discovered that have unusual magnetic properties. EuNi 5 P 3 has a series of steps in its magnetization curve 26 that result from a magnetic spin-flip transition in combination with thermally activated domain wall motion. A small Eu-Eu distance of 3.62 Å along the a-axis is believed to be responsible for the strong magnetic interactions observed 26 in EuNi 5 P 3 . EuAs 3 exhibits similar steps in its magnetization curve, and its magnetic phase diagram has been studied [27][28][29][30] by several groups; the temperature and field dependences of the lattice parameters are believed to be important in producing the steps in the magnetization. 31 Europium-containing Zintl phases, 32,33 such as Eu 14 InSb 11 and EuZnSn, typically exhibit Curie-Weiss paramagnetic behavior at higher temperatures and antiferromagnetic behavior at lower temperatures, typically below 25 K. There are some exceptions, such as Eu 3 P 2 and Eu 3 As 2 , which are both ferromagnetic semiconductors with Curie temperatures of 25 and 17.5 K, respectively. 34 Herein we report the structural, magnetic, and electronic properties of a new compound, Eu 3 InP 3 , which was initially produced serendipitously while investigating the crystallization of compounds with the Eu 14 InP 11 stoichiometry. Eu 3 InP 3 is the first magnetic compound that is isostructural with Sr 3 InP 3 . 35 It exhibits several unusual low-temperature magnetic ordering transitions.

Experimental Section
Synthesis. Large 3 × 1 × 1 mm 3 needle-shaped single crystals of Eu 3 InP 3 are formed from a reaction of elemental Eu/P/In in the ratio of 14:11:551, where indium is both a reactant and the flux. 36 All reactants and products were handled under a nitrogen atmosphere. Europium metal of 99.999% purity, obtained in ribbon form from the Ames Laboratory, was cut into small pieces, and red phosphorus, obtained from J. Matthey or Puratronic, was ground to small grains with an agate mortar and pestle. The europium and phosphorus were packed into an alumina crucible between layers of granular indium metal of 99.99% purity obtained from Aesar. A second crucible containing quartz wool was inverted over the reaction crucible, and the entire reaction vessel was sealed in a fused silica jacket that was back-filled with argon at 0.2 atm.
The reaction sequence involved heating the vessel to 500°C, holding for 1 h at 500°C, heating to 1100°C, holding for 6 h at 1100°C, slow cooling at 3°C/h to 850°C, holding for 19 h at 850°C, and then centrifuging at 850°C. The elemental ratios of 1:1:40 and 1:1:30 for Eu/P/In were also investigated but resulted in a mixture of crystals of Eu 3 InP 3 and a new phase identified as Eu 3 In 2 P 4 by single-crystal X-ray diffraction.
The reaction was also performed in sealed tantalum tubes, which were sealed in evacuated quartz ampules. Various reaction profiles were also investigated with the elements in the stoichiometric ratio of 3:1:3 for Eu/In/P. Dwell temperatures of 1100 and 1300°C for periods of up to 6 days were employed in these preparations. The product from these reactions was identified as Eu 14 InP 11 by X-ray diffraction.
All products were manipulated in a nitrogen-filled glovebox. The Eu 3 InP 3 crystals are extremely reactive, decomposing into a yellow powder upon exposure to air. X-ray Powder Diffraction. The Eu 3 InP 3 products were ground in a drybox to a powder with an agate mortar and pestle and placed between pieces of cellophane tape with ∼10% of NBS silicon standard. The Eu 3 InP 3 was transferred from the drybox to a Guinier camera, where the sample remained under vacuum for the duration of the data collection, which used Cu KR1 radiation. Film data from the samples were compared to powder patterns calculated from the single-crystal structure of Eu 14 InP 11 as well as from other known binary and ternary compounds. The powder diffraction pattern could be indexed according to the single-crystal structure of Eu 3 InP 3 , and no impurities were observed in the powder diffraction pattern.
Single-Crystal X-ray Diffraction. The Eu 3 InP 3 crystals were stored in Exxon Paratone-N oil and transferred to the crystallography laboratory in dry ice. A suitable crystal of 0.28 × 0.08 × 0.02 mm 3 volume was fixed to a glass fiber with silicon grease. A Siemens P3 diffractometer equipped with a modified Enraf-Nonius low-temperature apparatus and a graphite monochromator was used for data collection using Mo KR radiation. The crystals were centered and indexed, and the ψ-scan method was used to perform an empirical absorption correction. The structure was solved by direct methods and refined by the full-matrix least-squares method on F 2 (SHELXL-97, Sheldrick 1997). The structure was found to be isotypic with Sr 3 InP 3 . 35 The data were corrected for extinction and refined with anisotropic thermal parameters.
Conductivity Measurements. The resistance of Eu 3 InP 3 is so high that it exceeds the measurement limit of our four-probe instrument over most of the temperature range between 10 and 300 K. Thus, a two-probe measurement method was employed in which, in a nitrogen-filled drybox, Pt leads were attached to the crystal with Epoxy Technology H20E-175 silver epoxy. The sample was protected in an airtight jar when taken out of the drybox and was exposed in air for about 10 s when being placed in the measurement chamber. Temperature-dependent conductivity was obtained on a single crystal with a 0.3 × 0.1 × 0.2 mm 3 volume. The sample exhibited ohmic behavior. The measurement was reproduced on several different crystals. A Keithley model 617 programmable electrometer (200 GΩ limit) was used to measure the resistance. The current was selected automatically by the electrometer as the resistance of the sample changed to ensure measurement accuracy. The conductivity is calculated from the resistance measurement and the size of the crystal.
Magnetic Susceptibility Measurements. A Quantum Design MPMS superconducting quantum interference device (SQUID) magnetometer with a 7 T superconducting magnet was used to make dc magnetization measurements. A single crystal was placed in a drinking straw and secured with Paratone-N oil; several different crystals were studied. In addition, to ensure that the phase and crystallographic orientation were correct, single-crystal X-ray diffraction data were collected on a crystal after the magnetic measurements. The long axis of the crystal was confirmed to coincide with the b-axis of the unit cell. A powdered sample prepared from a number of crystals was also investigated. Zero field cooled data were collected from 2 to 300 K, and field cooled data were collected from 300 to 2 K. The data were collected utilizing the MultiVue software provided by Quantum Design. 37 Magnetization versus field data were obtained at fields up to 2 T at 2, 5, 8, 12, and 16 K.
Mo 1ssbauer Spectral Measurements. The europium-151 Mössbauer spectra were obtained on a constant-acceleration spectrometer that utilized a 151 m SmF 3 source. The absorber, which contained ∼60 mg/cm 2 of powdered sample mixed with boron nitride, was prepared and maintained under a dry nitrogen atmosphere. The spectra were measured in a Janis Super-Varitemp cryostat, and the temperature was controlled with a Lakeshore Cryogenics temperature controller with an accuracy of at least 1% of the observed temperature. The velocity scale was calibrated at room temperature with a cobalt-57 source and an R-iron foil. The isomer shifts are reported relative to EuF 3 at room temperature.
Heat Capacity. The heat capacity was measured on a Quantum Design physical property measurement system (PPMS) using a thermal relaxation method. The system utilizes a helium-3 refrigerator to reach temperatures below 2 K. Low-temperature data were obtained between 0.36 and 30 K on a 3.13 mg rectangular single crystal with applied magnetic fields of 0, 0.1, and 1 T. The crystal integrity was verified after the calorimetry measurement by magnetization. The Apiezon N-grease correction to the addenda for the heat capacity measurements was not applied to the field measurements, which may introduce an error as large as 1% in the sample heat capacity at temperatures below 10 K. Although the 0.1 T measurement may reflect a slight deviation from actual temperature values below 10 K due to the different thermometer calibration field factor utilized for this measurement, this deviation is negligible. The data points are extrapolated to zero temperature for the calculation of the entropy, which is obtained by integrating the specific heat divided by the temperature with respect to the temperature either from 0 to 30 K for the zero field or from 0 to 25 K for the applied field measurements.

Results and Discussion
Synthesis. Molten metal fluxes have been used to grow large single crystals for the characterization of physical properties. 36,38 Indium is an ideal flux because of its low melting point, relative inertness to the alumina reaction crucible, minimal wetting effects, and low vapor pressure. An indium flux was originally employed in an attempt to prepare large crystals of Eu 14 MnP 11 . Although this proved to be unsuccessful, a new compound, Eu 3 InP 3 , was obtained. This compound cannot be prepared from a stoichiometric mixture of the three elements sealed in tantalum tubes at the temperatures investigated. This suggests that the phase is peritectic or that competing thermodynamic phases are more stable. It is possible that Eu 3 InP 3 is a kinetic product that results when large amounts of indium are present.
Structure. Ternary Zintl compounds with the 3:1:3 stoichiometry typically crystallize in one of three structuretypes, two of which are orthorhombic with the Pnma space group 39,40 and the third of which is monoclinic with the Cmca space group. 39 All three structure-types possess chains of MPn 4 tetrahedra, but they differ in the connectivity of these tetrahedra. Eu 3 InP 3 is isotypic with Ca 3 InP 3 , 40 Sr 3 InP 3 , 35 and Ca 3 GaAs 3 , 40 all of which are composed of distorted [MPn 2 Pn 2/2 ] 6tetrahedra (where M is Ga or In and Pn is P or As) that are corner-shared to form linear chains. Because two of the Pn ions are shared by two tetrahedra, these two Pn ions are denoted as Pn 2/2 in the above formulation. The heavier antimony-containing compounds, Sr 3 GaSb 3 and Ba 3 GaSb 3 , have structural motifs based on chains of tetrahedra as well. Sr 3 GaSb 3 has nonlinear chains of corner-shared [GaSb 2 Sb 2/2 ] 6tetrahedra. 35 Large cations, in conjunction with antimony, result in compounds with the Ba 3 GaSb 3 structure which contains isolated [Ga 2 Sb 6 ] 12dimers composed of edge-shared tetrahedra. 40 In each of the structuretypes, the tetrahedral angles are distorted from the ideal values.
Each of the aforementioned structures can be rationalized with the Zintl concept. 35,40 The net charge of the Eu 3 InP 3 formula unit is zero when europium is divalent such that the compound may be formulated as containing three Eu 2+ , one In 3+ , and three P 3ions. Alternatively, the Zintl electron counting scheme yields (Eu 2+ ) 3 (4bIn -)(2bP -)(1bP 2-) 2 where b indicates the number of bonds to each element. Thus, formally, the In and P ions combine to form a polyanion, [InP 2 P 2/2 ] 6-, which counterbalances the charge of the three Eu 2+ cations.
Conductivity. The conductivity of Eu 3 InP 3 as a function of temperature is shown in Figure 4. Because the conductivity below 80 K is too low to be measured, its temperature dependence could only be obtained above 80 K. The temperature dependence of the conductivity indicates that Eu 3 InP 3 is a semiconductor with a room-temperature conductivity of 5.84(12) S/m. A plot of ln σ as a function of the inverse temperature is shown in the inset to Figure 4. By using the relationship a gap energy, E g , of 0.454(3) eV is obtained.
Magnetic Susceptibility Studies. Figure 5 shows the temperature dependence of magnetic susceptibility of Eu 3 InP 3 measured in a field of 0.010 T on a needle-shaped single crystal whose long needle b-axis was parallel with the applied magnetic field. The magnetic susceptibility can be fit with the Curie-Weiss law over the temperature range of 26 to 300 K, and it yields a Curie constant, C, of 22.70(4) (emu K)/mol and a Weiss temperature, θ, of 19.9(3) K. The effective magnetic moment, µ eff , calculated from the Curie constant is 13.47(1) µ B , a moment that is close to the 13.75 µ B value expected for three divalent europium ions. Below 15 K, several magnetic transitions are observed in Eu 3 InP 3 . More specifically, three peaks appear at 14, 10.4, and ∼5 indicating complex magnetic interactions.
To obtain more information about these three magnetic transitions, zero field cooled (ZFC) and field cooled (FC)

Complex Magnetic Ordering in Eu 3 InP 3
susceptibilities were collected at different applied magnetic fields between 2 and 20 K. Figure 6 shows the magnetic susceptibility of a crystal of Eu 3 InP 3 with its b-axis oriented parallel to the applied field. At an applied field of 0.001 T, the ZFC and FC susceptibilities are identical between 300 and 13 K. Below 13 K, they diverge and intersect with each other twice at ∼8.5 and 4 K. This behavior suggests the presence of a ferromagnetic or canted antiferromagnetic interaction below 13 K. The magnetic susceptibility obtained at 0.010 T is very similar to that obtained at 0.001 T, but the peaks in the susceptibility at 10.4 and 5 K are suppressed. Further, the temperatures of the susceptibility maxima shift compared with the 0.001 T data. More specifically, the 10.4 K susceptibility maximum moves to a lower temperature and the 5 K susceptibility maximum moves to a higher temperature. At 0.100 T, all the peaks are suppressed except for a small ZFC and FC divergence below 4 K. Figure 7 shows the same measurements as those in Figure 6 except that the field is applied perpendicular to the b-axis of the crystal. The results shown in Figure 7 are quite different from those in Figure 6, indicating the anisotropic nature of the magnetic transitions in Eu 3 InP 3 . Isotropically shaped crystals were also investigated and yielded the same results. In Figure 7, there are two obvious antiferromagnetic transitions at 10.4 and 14 K, consistent with the transitions observed with the field applied parallel to the b-axis; these peaks in the susceptibility are not suppressed even at an applied field of 0.100 T.
The crossing of the ZFC and FC susceptibilities observed below 8 K in Figure 6 suggests the presence of either complex magnetic interactions or domain motions in Eu 3 InP 3 . To investigate whether the magnetic transitions observed in Eu 3 InP 3 are caused by domain wall motion within the singlecrystal sample, ZFC and FC data were obtained on a powder sample of Eu 3 InP 3 . Grinding the sample into a very fine powder may disrupt the domain boundaries and thus decrease the domain effects. The results of this study, shown in Figure  8, indicate that the intersections of the susceptibility disappear and that the temperature dependence becomes simpler. This result supports the hypothesis that domain wall motion effects produce the complex transitions in the magnetic susceptibility below 8 K. However, because the complex behavior is observed mainly in the b-axis parallel direction, it is also possible that information is lost by averaging the various directions obtained in a powdered sample.
Heat capacity results, which will be discussed below, also indicate that the 5 K transition is an intrinsic magnetic transition. There are two reasons: first, there is a shoulder from 2 to 8 K in the heat capacity curve, and, second, the heat capacity curve measured between 2 and 8 K in a 1 T applied field is quite different from that obtained in the absence of a field.
Europium-151 Mo 1ssbauer Spectral Studies. The europium-151 Mössbauer spectra of Eu 3 InP 3 , obtained between 4.2 and 90 K, are shown in Figure 9. At 90 K, two absorption  regions are observed in the spectra: a major component at ∼ -10 mm/s and a minor component at ∼1 mm/s. The five spectra have been fit simultaneously to ensure a consistent temperature dependence of the hyperfine parameters.
The minor spectral component, which has a relative area of 5% and an isomer shift of 1 mm/s, has been constrained to be independent of temperature. This isomer shift is characteristic of europium(III) and indicates a small oxidation of the sample, perhaps on the surface of the particles. This minor absorption, which is a singlet at all temperatures, is assigned to an unknown europium(III) oxidation impurity that is not intrinsic to Eu 3 InP 3 .
The major spectral component at ∼ -10 mm/s indicates that europium is present as divalent europium in Eu 3 InP 3 . At 90 K, this spectral component is a broad singlet characteristic of paramagnetic europium(II), whereas, at 16 K, this component exhibits the onset of magnetic ordering of some of the europium(II) ions. In contrast, at 12, 8, and 4.2 K, the spectra clearly exhibit magnetic ordering of some or all of the europium(II) sites.
The major spectral absorption observed in Eu 3 InP 3 has been fit with three components with the same area assigned to the three crystallographically inequivalent europium sites. The line widths and the isomer shifts of these components have been fit but constrained to be independent of temperature. Because Eu 3 InP 3 is paramagnetic at 90 K, as is indicated by the magnetic susceptibility measurements shown above, the 90 K spectrum has been fit with three singlets. Between 16 and 4.2 K, the spectra have been fit with three octets whose hyperfine fields have been varied. The resulting spectral hyperfine parameters are given in Table 3.
Because all three europium sites in Eu 3 InP 3 have the same crystallographic degeneracies and rather similar environments, it is difficult to make an unambiguous assignment of the three Mössbauer spectral components to the three crystallographic europium(II) sites. The following tentative assignment (see Table 3) is proposed on the basis of the Wigner-Seitz cell volumes and the Eu-Eu average bond distances for the three crystallographically distinct europium sites. A Wigner-Seitz cell analysis, 42 which has been carried out with the 12-coordinate metallic radii of 1.99, 1.61, and 1.28 Å for Eu, In, and P, respectively, yields volumes of 40.561, 42.124, and 41.310 Å 3 for the Eu(1), Eu(2), and Eu(3) sites, respectively. The Eu-Eu average bond distances, obtained from the distances given in Table 2 for the near-   At 16 K, only small hyperfine fields of between 2 and 5 T are observed in Eu 3 InP 3 ; these fields may result from shortrange magnetic ordering slightly above the ordering temperature of 14 K. At 12 K, the Eu(1) and Eu(2) sites exhibit substantial fields of ∼17 T. Eu(1) is expected to order antiferromagnetically at the highest temperature, because of its short Eu-Eu average bond distance and because it forms dimers (see above) with the Eu(2) site, which also exhibits a substantial hyperfine field.
The Eu(3) site experiences a small transferred field at 12 K and is magnetically ordered only at 8 and 4.2 K. This Eu(3) site may experience some magnetic frustration resulting from the antiferromagnetic coupling of the Eu(1) and Eu(2) sites in the triangular arrangement of the three europium sites (see Figure 3). The existence of frustration would be compatible with the indication of ferromagnetic or ferrimagnetic interactions in the divergence between the zero field cooled and field cooled magnetic susceptibilities (see Figure 6).
Because the Mössbauer spectral experiments have been carried out in zero applied field, there may be some differences in the magnetic transition temperatures observed in the Mössbauer and the magnetic experiments. The magnetic transition observed at 14 K in Figure 4 may be associated with the ordering of the Eu(1) and Eu(2) sites observed in the 12 K Mössbauer spectrum, and the transition observed at 10.4 K in Figure 4 may be associated with the ordering of all three europium(II) sites in the Mössbauer spectra obtained at 8 K. The application of a magnetic field of 0.001 T may decrease the transition temperatures by up to 2 Ksit should be noted that the peak at 10 K in Figure 6 shifts to a lower temperature in an applied field of 0.010 T.
Heat Capacity. Figure 10 shows C p /T, normalized per mole of europium, as a function of temperature under different applied magnetic fields. The lattice contribution, which increases with temperature monotonically at low temperature, will be small so that we can attribute the nonmonotonic behavior of C p /T versus temperature to the magnetic contribution to the heat capacity. There are two obvious peaks at about 13.5 and 10 K, consistent with the two magnetic ordering transitions at 14 and 10.4 K observed in the magnetic susceptibility. These two transitions suggest that the interaction between the different divalent europium ions, the intersite interaction, may be weak, whereas the intrasite interaction is strong so that the crystallographically different sites order independently, as the Mössbauer spectral results indicate. The long tail above the 14 K transition temperature indicates the development of a short range order, which is also observed in the Mössbauer spectra. The ordering temperatures for both the 14 and 10.4 K transitions decrease in a 0.100 T applied field, although this decrease is very small for the 14 K transition. This is consistent with antiferromagnetic ordering at both transitions and suggests the possibility of a metamagnetic transition at a high magnetic field, which is clearly shown in the magnetization with respect to field data in Figure 11. The C p /T values obtained in a 1 T applied field show a broad peak near 14 K, which is consistent with a ferromagnetic transition in the applied field. Because the paramagnetic spins will be already partially aligned in this applied field, the transition is not as sharp as in the zero field and low field data. The corresponding entropy is shown as the solid lines in Figure 10. The Eu 2+ ground state, which is spherically symmetric, is weakly affected by the crystal field. Therefore, it is a reasonable approximation to consider the crystal field ground state as 8-fold degenerate. The entropy at 25 K, ∼R ln 8 per mole of europium, where R is the gas constant, indicates that this 8-fold degenerate europium state is consistent with the magnetic susceptibility results. As the temperature is  lowered below the transition temperature, the 8-fold degenerate state is depopulated through the magnetic ordering and entropy decreases. The total integrated entropy up through the 14 K transition is 2R ln 8. The missing R ln 8 entropy expected from the full ordering of the three europium ions per formula unit is contained in the fluctuations above 14 K and is not fully developed until about 25 K, an observation that is consistent with the Curie-Weiss temperature. Also consistent with this is the saturation magnetization at 5 K shown in Figure 11, where the full gJ z µ B ) 7 µ B per Eu ion develops. There exists approximately R ln 8 total entropy per mole at 8 K at the lower transition, indicating that there is still an 8-fold degeneracy present below the two transitions. This remaining degeneracy is lost in the broad hill-like feature in C p /T. Zeeman splitting of magnetic levels due to strong internal molecular fields could possibly explain the shape of this feature. Similar behavior has been reported in some Gd compounds, such as GdBa 2 Cu 3 O 7 and Gd 2 CuO 4 . 44,45 Magnetization Studies. Hysteresis curves have been measured, and the magnetization as a function of the field at 5 K obtained with the single crystal b-axis of Eu 3 InP 3 parallel to the applied field is shown in Figure 11. The analogous perpendicular magnetization results are available in the Supporting Information. In the perpendicular direction, the magnetization increases linearly below 0.2 T, whereas above 0.2 T it starts to saturate. The parallel curve saturates at 1 T with a saturation moment of 20.777(8) µ B per formula, which is consistent with the 21 µ B expected for the three Eu 2+ ions. The inset to Figure 11 shows the magnetization obtained between (0.3 T. The sharp magnetization increase with the applied field between 0.05 and 0.1 T suggests a spin-flip transition. This is consistent with the heat capacity measurements (see Figure 10) which reveal that the observed C p /T is very different when measured at 0 and 0.1 T versus at 1 T.
The parallel-direction magnetization obtained at 2, 8, 10, and 12 K is similar to the 5 K results shown in Figure 11. At high temperature, the magnetization is harder to saturate than at low temperature. At 16 K, which is above the magnetic transition temperature, the magnetization is linear up to ∼0.3 T and then begins to saturate. This can be explained by a local magnetic correlation between magnetic sites above the long range ordering temperature that is observed in the Mössbauer spectrum obtained at 16 K. Further, because the Weiss temperature, θ, obtained from the Curie-Weiss fit is positive, this correlation is believed to be ferromagnetic.
Because Eu 3 InP 3 is a semiconductor, two possible mechanisms may explain the magnetic transitions: super-exchange and spin-spin exchange. However, super-exchange is expected to be minimal because, although the Eu-P bonds may have a strong covalent character, the f-orbitals of europium contribute very little to the bonding. Hence, spin-spin exchange is expected to be the dominant exchange interaction in Eu 3 InP 3 . Figure 3 shows the europium lattice along with some of the Eu-Eu distances. The shortest distance of 3.5954(7) Å is expected to provide the strongest interaction. All other distances are quite similar, ranging from 3.833 to 4.086 Å, and the interactions are expected to be much weaker. If Eu(1) and Eu(2) order antiferromagnetically first, as indicated by the Mössbauer spectral measurements, then one might expect complex or "frustrated" magnetic behavior for Eu(3) based on the triangular arrangement of the three divalent europium ions.
Summary. Eu 3 InP 3 is a magnetic, semiconducting Zintl compound. There are three different Eu 2+ sites, which interact in a complex fashion. Magnetic susceptibility, Mössbauer, and heat capacity studies indicate that there are three magnetic transitions at 14 K, 10.2 K, and ∼5 K. A spin-flip transition can be seen in the magnetization curve at 5 K with an applied field of 0.05 T.