High-pressure and high-temperature synthesis and physical properties of Ca2CrO4solidL. P. Cao, M. L. Jin, W. M. Li, X. C. Wang, Q. Q. Liu, Y. L. Xu, L. Q. Pan, and C. Q. Jin Citation: AIP Advances 6, 055010 (2016); doi: 10.1063/1.4949008 View online: http://dx.doi.org/10.1063/1.4949008 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/6/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Multiferroic properties of La-doped Bi2FeCrO6 prepared by high-pressure synthesis J. Appl. Phys. 111, 07C702 (2012); 10.1063/1.3670576 Influence of global magnetic state on chemical interactions in high-pressure high-temperature synthesis of B2 Fe 2 Si Appl. Phys. Lett. 94, 181912 (2009); 10.1063/1.3131784 On the structure of high-pressure high-temperature η -O 2 J. Chem. Phys. 130, 164516 (2009); 10.1063/1.3118970 High-pressure Raman scattering study on zircon- to scheelite-type structural phase transitions of R CrO 4 J. Appl. Phys. 103, 093542 (2008); 10.1063/1.2909202 High-pressure high-temperature synthesis and magnetic properties of ordered perovskite Sr 2 Cu ( Re 0.69 −x W x Ca 0.31 ) O 6 ( 0 x 0.6 ) J. Appl. Phys. 101, 09N501 (2007); 10.1063/1.2672395

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AIP ADVANCES 6, 055010 (2016)

High-pressure and high-temperature synthesisand physical properties of Ca2CrO4 solid

L. P. Cao,1,2 M. L. Jin,2 W. M. Li,2 X. C. Wang,2 Q. Q. Liu,2 Y. L. Xu,1,3

L. Q. Pan,3,a and C. Q. Jin2,a1Department of Physics, University of Science and Technology Beijing, Beijing 100083, China2Key Laboratory of Extreme Conditions Physics, Institute of Physics, Chinese Academyof Science, Beijing 100190, China3College of Science, China Three Gorges University, Yichang 443002, China

(Received 3 March 2016; accepted 25 April 2016; published online 5 May 2016)

The bulk Ca2CrO4 samples were synthesized under high pressure and high temper-ature conditions using CaO and CrO2 as starting materials. The structure of the pre-pared Ca2CrO4 solid is characterized by X-ray diffraction with Rietveld refinementas tetragonal structure with the space group I41/acd. The CrO6 octahedrons elongatealong c axis and rotate in ab plane. DC and AC magnetic susceptibility measurementresults indicate spin glass behavior at low temperature. Temperature dependenceof resistivity measurement results show Ca2CrO4 is an insulator at both ambientcondition and high pressure. C 2016 Author(s). All article content, except whereotherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4949008]

I. INTRODUCTION

Single-layered perovskites A2MO4 (A is alkaline/rare earth element and M is transition metalelement) exhibit rich physical properties which attract much attention, such as high temperatureCu-based superconductor La2−xSrxCuO4

1,2 and orbital/charge ordering in La0.5Sr1.5MnO4.3–5 In thestrongly correlated electronic systems, spin, charge and orbital coupled each other which leadingto very interesting phenomenon.6 Sr2VO4 with 3d1 electronic configuration and antiferromagneticground state shows an apparently orbital ordering transition at Tc about 100K, while muon spinrotation and relaxation (µ+SR) measurement shows that static long-range antiferromagnetic orderoccurred below TN=8K.7,8 The separation of spin ordering at TN and orbital ordering at TC bringa problem to theoretical calculation in which spin-orbital coupling was considered to play impor-tant role in the magnetic ground state.9,10 For the 3d2 systems, both Sr2CrO4 and LaSrVO4 werereported. Despite the crystal structure and the electronic configuration are similar, the two com-pounds exhibit quite different magnetic ground states. High pressure phase Sr2CrO4 with K2NiF4structure is antiferromagnetic with TN=112K, both magnetic susceptibility and µ+SR measurementsindicate the long-range magnetic ordering.11,12 While LaSrVO4 was regarded to be a rare candidatefor the spin-orbital liquid state related to t2g orbitals, reported by Dun et al. Till now, spin-orbitalliquid systems are mostly eg orbital related. For example, spin glass liquid state LiNiO2 induced bygeometrically frustrated lattice13,14 and Ba3CuSb2O9 caused by strong spin-orbital entanglements ofCu2+ ion.15,16 LaSrVO4 shows a similar K2NiF4 structure and a weak structure distortion presentedaround 100K accompanied by orbital fluctuation in a broad temperature range. No long rangemagnetic ordering was found as far as down to 0.35K, but a short range magnetic order revealsat 11K by susceptibility measurement. The AC susceptibility was frequency-independent and themagnetic contribution of the specific heat shows T2 behavior below 10K, which can exclude thatthe transition was a spin-glass behavior. The density functional theory calculation without orbitalfluctuation effect gives a magnetic order ground state. According to the above studies, the mag-netic frustration in LaSrVO4 was considered to be resulted from the orbital fluctuations induced

aCorresponding authors: lpan@ctgu.edu.cn & Jin@iphy.ac.cn

2158-3226/2016/6(5)/055010/7 6, 055010-1 ©Author(s) 2016.

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by structure distortion.17 The magnetic order changed with the structure is attractive. Compared toSr2CrO4 and LaSrVO4, another 3d2 electronic configuration compound Ca2CrO4 should be moredistorted in crystal structure due to the small ion radius of Ca2+, which probably suppresses longrange magnetic order and leading to spin-orbital liquid or spin glass behavior. In this article, theCa2CrO4 compound was synthesized under high pressure and high temperature and the electricaland magnetic properties of the sample were measured.

II. EXPERIMENT

Polycrystalline Ca2CrO4 sample was synthesized under high pressure and high temperaturecondition. CaO and CrO2 were used as starting materials. Stoichiometric reactants were pressed intopellets after thoroughly mixed in glove box. Then the pallet was sealed with gold foil and reactedunder about 5.5 GPa and 1000 ◦C for 30 min. The pressure was released after temperature quenchedto room temperature. The structure and lattice parameters were determined by X-ray diffraction(XRD) solved by Rietveld refinement using GSAS+EXPGUI software package.18 DC magneticsusceptibility was measured by superconducting quantum interference device (SQUID); AC suscep-tibility, electric resistance and specific heat were measured by physical property measurementsystem (PPMS). Resistance under high pressure was measured using diamond anvil cell (DAC) inMaglab system.

III. RESULTS AND DISCUSSION

XRD pattern and Rietveld refinement result is shown in Fig. 1. There is about 4.24% of residualCaO reactant in mole ratio according to the refinement. The Ca2CrO4 crystal structure was refinedby taking Ca2MnO4 with space group I41/acd as the prototype.19 The refinement gives a satisfiedresult with Rp=0.0506, wRp=0.0746 and χ2 = 4.946. The lattice parameters and other detailedinformation are shown in Table I. There are two oxygen sites in CrO6 octahedron: apical O1 andequatorial O2, as shown in the crystal structure schematic in Fig. 1. The bond length of Cr-O1 islonger than that of Cr-O2, indicating an elongation of the octahedron along c axis. The bond lengthratio of Cr-O1/Cr-O2 is about 1.114, which is larger than that in LaSrVO4, leading the t2g orbitalssplitting into one higher energy dxy orbital and two degenerate lower energy orbitals dxz/dyz. Theorbital degree of freedom is suppressed due to the two 3d electrons of Cr4+ tending to occupy thedxz/dyz orbitals. Additionally there is a 9 ◦ rotation of the octahedrons in ab plane due to the smallradius of Ca2+, the rotation directions are opposite between two successive octahedral layers.

Temperature dependence of magnetic susceptibility of Ca2CrO4 with the applied field of 10kOeis shown in Fig. 2. The inset shows Curie-Weiss fit above 160K, giving an effective magnetic

FIG. 1. Powder XRD Rietveld refinement result and the crystal structure of Ca2CrO4.

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TABLE I. Structure detailed information of Ca2CrO4. (Rp=0.0506, wRp=0.0746, χ2= 4.946).

Atoms Site x y z

Ca 16d 0 0.25 0.5498(1)Cr 8a 0 0.25 0.375O1 16d 0 0.25 0.4603(2)O2 16f 0.2102(5) 0.4602(5) 0.125

Cr-O1: 2.076(4) Å Cr-O2: 1.8636(6) Å Cr-O2-Cr: 161.9(2) ◦

Space group: I41/a c d; a=5.2055(1) Å; c=24.3288(2) Å; V=659.24(1) Å3

moment µeff = 4.7 µB and Weiss constant θ = -1026 K. The effective magnetic moment which islarger than the theoretical value of localized Cr4+ with two 3d electrons (2.83 µB) and the largeWeiss constant which is due to the weak temperature dependence of susceptibility indicate that thesample does not obey Curie-Weiss law and the non-localized electronic state. Curie-Weiss fit above200K for Sr2VO4 gives the µeff = 5.0 µB and Weiss constant θ = -1100 K,7 which was similar toCa2CrO4. The weak temperature dependence of susceptibility in Sr2VO4 was owing to the stronglyenhanced Pauli paramagnetism which is typical for the strong correlated 3d electron systems, espe-cially near the crossover of insulator-metal transition.7 The reciprocal of Ca2CrO4 susceptibilitydeviates from linear behavior below 160K, indicating the presence of spin fluctuation. A suscepti-bility peak was observed in ZFC curve at low temperature with the apparently divergence betweenZFC and FC, and this transition temperature decreased with increasing the applied field, as shownin the inset of Fig. 3. Field dependence of magnetization shows that evident magnetic hysteresispresents only below the transition temperature. To further understand the mechanism of the transi-tion, both AC susceptibility and specific heat of the sample were measured. AC susceptibility wasmeasured under 1 KOe applied field for varies frequencies, as shown in Fig. 4. With the increasingof frequency, the transition temperatures increase gradually and the magnetization become smaller,which is corresponding with the characteristic of spin glass behavior. The Vogel-Fulcher law couldbe used to describe spin glass behavior as ω = ω0 exp [−Ea/kB (TF − T0)], where ω is the frequency,TF is the freezing temperature. Linear relationship between TF and 1/ ln (ω0/ω) is obtained. Settingω0 = 1013Hz, which is a typical value for the spin glass system,20,21 TF dependence of 1/ ln (ω0/ω)is shown in Fig. 4 inset. The linear fitting gives T0 = 3.78K and Ea/kB = 75.1. Fig. 5 shows thetemperature dependence of specific heat of the sample. The inset shows T2 dependence of Cp/Tand linear fitting was done above the magnetic transition temperature. The fitting curve gives the

FIG. 2. Temperature dependence of magnetic susceptibility curves under 10KOe applied field and Curie-Weiss fit at hightemperature range of ZFC data (inset).

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FIG. 3. Field dependence of magnetic moment measured at different temperatures. The inset shows temperatures correspondto maximum susceptibility under different applied fields of ZFC.

slope 8.845*10−4mJg−1K−4 and from which the Debye temperature was calculated to be ΘD=428K.No apparently transition was found in specific heat measurement around the magnetic transitiontemperature, and the specific heat of the sample did not change with applied fields. The featuresmentioned above indicate that Ca2CrO4 exhibits spin glass magnetic behavior.22,23

Temperature dependence of electrical resistivity of the sample exhibits an insulating behavioras shown in Fig. 6, the data below 70 K was absent for out of measurement limit. Consideringthe nearest-neighbor hopping conduction at the relatively high temperature range, the resistivityρ = ρ0 exp(∆/kBT), where ρ0 is a constant and ∆ is the activation energy. According to the linearfitting of ln ρ∼T−1 for temperature range 230∼300K, shown in Fig. 6 inset, activation energywas estimated to be ∆ = 0.086 eV. This value is smaller than that in Sr2CrO4.11,24 For Sr2CrO4

single-crystalline film, the electronic structure was investigated by optical conductivity spectra. Theresult shows Sr2CrO4 is Mott-Hubbard insulator dominated by the Mott-Hubbard transition in abplane with an optical gap 0.3eV.25 Ca2CrO4 has the shorter Cr-O2 bond length and a 9 ◦ rotationof the octahedrons in ab plane, the Mott-Hubbard transition energy should be lower than Sr2CrO4.With the decreasing of the temperature, ln ρ∼T−1 deviated from linear relationship gradually, Mottvariable-range hopping (VRH) conductivity would dominate the electrical transport. The VRH

FIG. 4. The real part of AC susceptibility under 1 KOe applied field.

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FIG. 5. Specific heat of Ca2CrO4; the inset shows the Cp/T∼T2 fitting above transition temperature.

resistivity ρ = ρ′0 exp(T0/T)1/(n+1), where ρ′0 and T0 are constants and n is related to the dimension-ality of the conduction mechanism. As the insets of Fig. 6 show, both n=2 and 3 give good linearfitting at the whole temperature range. Despite Ca2CrO4 has the two dimensional layered structure,the electrical transport property is affected possibly by the lattice distortion. Resistance of Ca2CrO4under high pressure was measured using DAC. As Fig. 7 shows, resistance decreased with thepressure increasing. Insulator-metal transition was not occurred up to 43 GPa.

Compared with the compounds of Ca2CrO4 and Sr2CrO4, the simple perovskites of CaCrO3and SrCrO3 with CrO6 octahedral coordination are located in the crossover of metal-insulator tran-sition and present much lower resistivity.26–29 The infrared reflectivity measurement suggests thecompounds of CaCrO3 and SrCrO3 are metallic,30,31 while for the resistivity measurement they arereported to be insulators and insulator-metal transition happens under high pressure,26,32,33 whichis not observed in Ca2CrO4. The orthorhombic distorted CaCrO3 is more insulating than cubicSrCrO3 with a narrower band width,26 while more distorted Ca2CrO4 is more metallic than Sr2CrO4,indicating the different electric transport mechanism. Recent research on CaCrO3 indicates that one3d electron is localized in the dxy orbital and another is itinerant electron in dxz/dyz orbital. The dxyelectron delocalized gradually under high pressure, leading CaCrO3 from a Mott insulator to bandinsulator and finally become metallic.34 The CrO6 octahedrons in CaCrO3 is compressed along c

FIG. 6. Temperature dependence of resistivity under 0 applied field. The insets show the fitting with nearest-neighborhopping model at temperature range 230∼300K (left bottom) and variable-range hopping model for n=2 and 3 at the wholetemperature range (right top).

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FIG. 7. Temperature dependence of resistances under high pressure.

axis,30,35 while it is elongated in the layered Ca2CrO4 and Sr2CrO4, which leads to quite differentorbital and spin distribution. The insulating behavior of the layered perovskites possibly due to thesuppression of orbital degree of freedom and the two dimensional crystal structure.

In the single-layered perovskites with 3d2 electronic configurations, Sr2CrO4 presents longrange antiferromagnetic order, LaSrVO4 shows a possible spin-orbital liquid state without longrange magnetic order and Ca2CrO4 exhibits spin glass behavior with a quite low frozen temperature.The melting of long range magnetic order in LaSrVO4 was due to the orbital fluctuations whichresult from the presence of local ordered clusters of Jahn-Teller distorted VO6 octahedrons. Thefrustrated orbital degree of freedom can lead to orbital fluctuations in some systems.17 The largeeffective moment and Weiss constant in Ca2CrO4 might suggest the strong fluctuations of the elec-trons. The spin and orbital fluctuations resulted from the structure distortion suppress the long rangemagnetic order and lead to the spin glass behavior in Ca2CrO4, which is similar to LaSrVO4. Tofurther confirm the magnetic mechanism, SrxCa2-xCrO4 series solids are the good candidates to beused in the exploration of the transition from long range antiferromagnetic order Sr2CrO4 to spinglass Ca2CrO4.

IV. CONCLUSIONS

Ca2CrO4 was synthesized using high pressure and high temperature technique. The refine-ment of powder XRD data shows that Ca2CrO4 is tetragonal with distorted CrO6 octahedrons.Curie-Weiss fit of DC susceptibility measurement of the samples gives quite large effective momentand Weiss constant, indicating non-localized electronic state and spin fluctuation. DC and ACsusceptibility measurement results show spin glass magnetic behavior. The melting of long rangemagnetic order is attributed to spin and orbital fluctuations induced by the structure distortion. Theelectrical insulating behavior observed under both ambient pressure and high pressure of up to43GPa resulted from the suppression of orbital degree of freedom and the two dimensional crystalstructure.

ACKNOWLEDGEMENTS

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