Unconventional metallic behavior and superconductivity in the K-Mo-O system

Transport properties and magnetization measurements of the K x MoO (cid:2) x (cid:2) 0.25 (cid:2) compound are reported. The compound crystallizes in the oxygen deﬁcient MoO 2 monoclinic structure with potassium atoms occupying interstitial positions. An unconventional metallic behavior with power-law temperature dependence is related to a magnetic ordering. Superconducting transition with small volume fraction is also observed near 7 K for a sample with low potassium composition.


I. INTRODUCTION
Interplay between superconductivity and magnetism has excited the scientific community due to the possibility to understand unconventional electron-pairing mechanisms. [1][2][3][4][5] The recent discovery of the iron-based superconductors with high critical temperature 1,6 and rich temperature-composition phase diagrams 3,7,8 has offered an unique opportunity for studying a possible magnetic-ordering-mediated hightemperature superconductivity. [1][2][3]9 Furthermore, the existence of superconductivity in unconventional metals such as Li 0.9 Mo 6 O 17 , 10,11 Na x CoO 2 , 12 and cuprates 9,13 is still a challenge to be understood.
In all of these compounds the existence of anisotropy seems to play an important role for the appearance of superconductivity. 10,[14][15][16] One of the most important mechanisms for the high anisotropic conductivity was predicted by the Luttinger liquid ͑LL͒ theory. 17 In this model no discontinuity is expected in the momentum distribution at the Fermi surface. 17 Furthermore, spin-charge separation and powerlaw temperature ͑T͒ dependence of the correlation functions are key features of the LL physics. 17 In cuprate superconductors, possible LL behavior is controversial and remains under discussion. 18 However, in the purple bronze Li 0.9 Mo 6 O 17 compound the quasi-one-dimensional ͑1D͒ conductivity has been unambiguously related to the appearance of the superconducting state. 10,14 Recently, the normal-state electrical resistance of the Li 0.9 Mo 6 O 17 compound has been properly described by a two-band LL with two power-law T terms. 10 Due to this power-law T dependence, 10 high anisotropic ratio in the electrical conductivity 15,19 and thermal expansion 14 along crystallographic axes, photoemission experiments, 20 and band-structure calculations, 21 the Li 0.9 Mo 6 O 17 compound is nowadays recognized as the best example for the LL behavior.
Despite the excellent agreement with the LL physics provided by the Li 0.9 Mo 6 O 17 compound 10,14,20,21 many other molybdenum phases have been considered for the study of lowdimensional behavior. 19,[22][23][24][25][26] Basically, Mo compounds have Mo-Mo or Mo-O-Mo channels along one axis providing high anisotropic conductivity necessary for quasi-1D behavior. 14,15,19,[26][27][28] One difficult task for studying Mo compounds is related to the preparation of high-quality samples ͑single and polycrystals͒ with workable sizes. This is primarily due to the difficulty to control the stoichiometry since formation of volatile MoO 3 is common at the temperatures used during sample preparations. [29][30][31] Furthermore, many oxidation states of the Mo atoms could play important role for the formation of Mo compounds with particular physical properties. 31 Based upon above mentioned questions, searching for new unconventional metals exhibiting superconductivity could be very important for clarifying possible unconventional electron pairing mechanisms. In this paper we report on the crystalline structure, transport properties, and magnetization measurements of a phase K x MoO 2−␦ . An unconventional metallic behavior with power temperature dependence has been observed. Sample with low potassium composition has shown superconductivity near 7 K. The interplay between the unconventional metallic behavior and magnetic ordering is also discussed.

II. EXPERIMENTAL
Polycrystalline samples were prepared as pellets using high purity K 2 CO 3 , MoO 3 , and Mo powders. The precursor K 2 MoO 4 was obtained after heat treating K 2 CO 3 and MoO 3 in air at 400°C for 24 h followed by a treatment at 700°C for 24 h. Polycrystalline samples of K x MoO 2−␦ ͑0 Յ x Յ 0.25͒ were sintered between 650 and 750°C for 3 days in quartz tube under vacuum after compacting appropriate amounts of K 2 MoO 4 , MoO 3 , and Mo powders. Different heat treatments resulted in different potassium ͑x͒ and oxygen content due to the volatilization of some atoms inside quartz tubes. MoO 2 samples were prepared using a similar procedure. Compositions of some samples were verified by flame atomic absorption spectrometry ͑FAAS͒ analyzes. Standard deviations of 3% and 4% were observed in the FAAS measurements for K and Mo atoms, respectively. X-ray diffractometry and simulations were performed using Cu K␣ radiation and POWDER CELL program, 32 respectively. Electrical resistance measurements as a function of temperature were carried out in a MagLab Oxford or physical properties measurement system ͑PPMS͒ system from 2 to 300 K. Magnetization measurements were performed using vibrating-sample magnetometer coupled in a PPMS or superconducting quantum interference device. Table I shows crystallographic data used in the simulations of x-ray powder-diffraction patterns for the K x MoO 2−␦ compound.

III. RESULTS
Experimental and simulated x-ray diffractograms 32 for the K x MoO 2−␦ compound are shown in Fig. 1. They agree each other and are very similar to that of the MoO 2 . 33 Only slight displacements of diffraction peaks are observed in MoO 2 potassium-doped samples. This can be clearly observed for the peaks shown in Fig. 2. The displacements to lower angles imply increasing of the lattice parameters which can be attributed to the increasing of the potassium composition in the K x MoO 2−␦ compound.
Based upon the crystallographic data, the crystal lattices of both MoO 2 and K x MoO 2−␦ monoclinic structures were simulated. Crystalline structures are shown in Fig. 3.
MoO 2 forms a distorted rutile structure having Mo-Mo metallic bonds along a axis of the monoclinic structure. [34][35][36] Due to those Mo-Mo bonds and other structural aspects, MoO 2 is supposed to show anisotropic behavior. Structural and electronic properties of this dioxide remain under investigation. 34,37 For the K x MoO 2−␦ compound, simulations of x-ray powder diffractograms suggest that potassium atoms occupy interstitial positions of the oxygen deficient MoO 2 as indicated on the right panel of the Fig. 3. Possible interstitial Wyckoff positions are 2a, 2b, 2c, and 2d. However, the simulations suggest that the most probable position is 2d. Other positions are unlikely because potassium atoms would occupy interstitial positions closer to the molybdenum atoms. Furthermore, potassium atoms occupying 2d sites have another importance since none of them occupy positions between Mo-Mo bonds along a axis.
Based upon starting sample compositions and atomic absorption spectroscopy, the K x MoO 2−␦ compound has potassium composition between 0.05 and 0.25 and oxygen contents ranging from 1.    Table I. have similar diffraction patterns nor physical properties as follow. In Fig. 4͑a͒ is displayed the electrical resistance as a function of temperature, R͑T͒, for the K 0.14 MoO 1.58 sample.
The compound shows a metal-like behavior. An anomalous metallic behavior can be noticed below ϳ70 K in which R͑T͒ reduces at low temperatures instead showing a typical saturation or linear T dependence as for conventional metallic conductors. 42 Similar behaviors have been found for several samples with different K compositions in the 0.05 Յ x Յ 0.25 range ͓see upper inset of the Fig. 4͑a͒ and main panel of the Fig. 4͑b͔͒. Near T S = 250 K, the K 0.14 MoO 1.58 sample displays a phase transition. Results for several samples show that this transition is very sensitive to the potassium composition. Hysteresis near 225 K in the R͑T͒ curves for the K 0.25 MoO 1.5 sample, shown in inset of the Fig.  4͑b͒, suggests that the transition is a first-order phase transition. In lower inset of the Fig. 4͑a͒ one can see the conventional linear temperature dependence for a MoO 2.01 sample 43,44 which is displayed together with the anomalous behavior of the K 0.14 MoO 1.58 . The results unambiguously demonstrate the relevance of K doping or oxygen deficiency on the appearance of the anomalous metallic behavior at low temperatures.
We have observed that a simple power law, R͑T͒ϳT ␣ , with ␣ = 0.53Ϯ 0.07 has fitted the results for several K x MoO 2−␦ samples with 0.05Յ x Յ 0.25. The origin of this power-law temperature dependence is under investigation but could indicate a possible quasi-1D character of the K x MoO 2−␦ compound despite the possible extrinsic intergranular effects on the electrical transport. This behavior is similar to the discussion reported recently for the Li 0.9 Mo 6 O 17 , 10,14 however the power-law temperature dependence for compound reported here has only one power-law term with positive ␣ exponent. 14,18 Furthermore, this unconventional behavior could be in agreement with the anisotropic behavior expected to the Mo-Mo channels along a axis of both MoO 2 and K x MoO 2−␦ compounds ͑see Fig. 3͒.
Magnetization measurements in zero-field-cooled ͑ZFC͒ and FC procedures, shown in Fig. 5͑a͒, suggest that the unconventional metallic behavior is related to a magnetic ordering below T M Ӎ 70 K. The M͑T͒ curves in ZFC and FC for the K 0.3 MoO 2 sample are displayed in the inset of the Fig. 5͑a͒. No magnetic ordering is observed near the firstorder phase transition. The increasing of the magnetic moment at lower temperatures and the hysteresis shown in Fig.   ! "  5͑b͒ suggest an unusual weak ferromagnetic ordering below T M . We have noticed that the general magnetic and electrical behaviors of the K x MoO 2−␦ samples are very similar to those reported for the Na x CoO 2 compound with x = 0.75 and 0.82. 12,45 The origin of the similarities is under investigation.
Finally, let us turn our discussion to the observation of superconductivity in low K-doped sample. Figure 6 displays magnetization measurements for the K 0.05 MoO 1.63 sample with rod shape in order to reduce demagnetization effects. Figure 6͑a͒ shows the magnetization as a function of temperature in the ZFC and FC procedures. A clear superconducting transition can be observed near 6.5 K for 200 Oe. This is the highest superconducting critical temperature reported for the K-Mo-O system up to now. Only one previous report suggests superconductivity near 4.2 K in this system. 46 No drop in the resistance as a function of temperature has been observed ͑see inset of Fig. 6͒, indicating that the sample has low superconducting volume fraction. 47 Estimation of the superconducting volume fraction based upon the magnetization measurement shown in Fig. 6 gives 4% which is much lower than that expected for the percolation limit. 47,48 In Fig. 7͑a͒ one can see the magnetization versus applied field measured at different temperatures below T c . Clear Meissner and mixed states can be noticed. Based upon the points in the M͑H͒ curves indicated by arrows for different temperatures, one can find the H C1 and H C2 lines shown in Fig. 7͑b͒ using the H Ci ͓1−͑T / T C ͒ 2 ͔ equation with i = 1 or 2. From the fittings it is possible to estimate the critical temperature at zero-magnetic field as 7.25 K. H C1 and H C2 at zero temperature are also estimated to be 450 Oe and 802 Oe, respectively. Those values provide ͑0͒ = 641 Å, ͑0͒ = 1210 Å, and = 1.89, indicating that the K x MoO 2−␦ compound is a type-II superconductor. 49

IV. CONCLUSION
A phase in the K-Mo-O system with K x MoO 2−␦ composition is reported. An anomalous metallic behavior with power-law temperature dependence has been found for several samples in 0.05Յ x Յ 0.25 range. The anomaly with an exponent ␣ ϳ 0.5 is related to a magnetic ordering below 70 K. Small superconducting volume fraction has been observed for a sample with low K doping. Correlation between the unconventional metallic behavior and the magnetic ordering below T M in closely related superconducting sample is under investigation.  FIG. 6. ͑Color online͒ Magnetization as a function of temperature for the K 0.05 MoO 1.63 sample. A superconducting transition can be observed near 6.5 K for 200 Oe. In inset the R͑T͒ behavior of the sample is displayed. No drop in R͑T͒ is observed near the diamagnetic superconducting transition. Due to the low superconducting fraction of the sample, the power-law temperature dependence remains responsible for the electrical behavior. Arrows indicate the H C1 ͑T͒ and H C2 ͑T͒. ͑b͒ T C at zero field, and H C1 and H C2 at zero temperature were determined as the best fitting parameters using the H Ci ͓1−͑T / T C ͒ 2 ͔ equation with i = 1 or 2. H C1 ͑0͒ and H C2 ͑0͒ allow one to estimate ͑0͒, ͑0͒, and as indicated.