Magnetic susceptibility and electrical properties of VSe2 single crystals

Solid State Communications, Vol. 20, pp. 251—254, 1976.

Pergamon Press.

Printed in Great Britain

MAGNETIC SUSCEPTIBILITY AND ELECTRICAL PROPERTIES OF VSe2 SINGLE CRYSTALS C.F. van Bruggen and C. Haas Laboratory of Inorganic Chemistry, Materials Science Center of the University, Groningen, The Netherlands (Received 18 June 1976 by A.R. Miedema) Magnetic susceptibility, in-plane resistivity and Hail effect data of VSe2 single-crystal plates are reported. These data exhibit anomalies at 100 K. The occurrence of these anomalies is presumably due to Fermi-surface changes resulting from the onset of a charge density wave instability. DURING A STUDY of the magnetic and structural phase transitions in the non-stoichiornetric sodium intercalates Na~VSe 2 it appeared that no physical properties of the “host” compound VSe2 were known from literature, apart from some rough susceptibility measurements on powder samples with composition VSe1% and VSe2~~2 In the course of the investigations on sodium intercalates’ some physical properties of pure VSe2 were also measured. Because VSe2 recently has aroused interest by the assumption of a charge density wave occurring at some low tern34 (CDW) it seemsinstability worthwhile to report of peratures, these physical properties obtained on well defined single crystals. The idealised crystal structure of this metallic 3d1 layered vanadium diselenide is of the Cd(OH) 2 type, space group P3m, with lattice constants a = 3.35 A the and 2 In this structure cV=atoms 6.10 A at room temperature. are in trigonally distorted octahedra of Se, which are elongated (—‘ 10%) in the direction of the trigonal c-axis. VSe 2 single crystals were prepared by the chemical transport method with selenium vapour as a transport agent (temperature time one week, Se gradient 870 780°C,transport 5 The grown 2 pressure 3.5 atm). crystals were relatively large but thin hexagonal plates with a black metallic luster and dimensions typically of about 10 x 10 x 0.05mm3. The composition determined by chemical analysis revealed a small excess of vanadium with respect to the ideal stoichiometry, i.e. V 1~Se2 with ~ = 0.015, corresponding to the selenium-rich phase boundary of VSe 2 during the crystal growth. Magnetic-susceptibility data of VSe2 single crystals and samples were obtained from of 2 K to roompolycrystalline temperature using a Faraday-type system Oxford Instruments Company Ltd. equipped with a superconducting magnet with separate gradient coils, and a Mettler ME2 1 electronic vacuum micro balance with a lowest weighing range of 1 mg force and 0A pg sensitivity. Polycrystalline VSe 2 was also measured be,~

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251

tween room temperature and 1000 K, with a 6Faraday For the balance which has been described previously. single-crystal measurements a stacking of crystal plates was used in a special quartz sample holder that could be rotated over 90°with respect to the magnetic field. Both the temperature and field dependences for the empty sample holder in the parallel and perpendicular position were accurately calibrated. The principal susceptibilities parallel and perpendicularto the trigonal c-axis are indicated as XIIc and y~respectively, and their H temperature dependences forFig. an applied field = 12.40 kOe are shown in 1 (solidmagnetic lines). The anisotropy of these principal susceptibilities appears to be small; XIic is somewhat larger than y~,with values of 315.1 x 10—6 and 293.5 x lO6emu/mole resp. at room temperature. Both susceptibilities show anomalies around K. Atand this temperature Xii and resp., y~have of 338 x9510-6 325 x lO6emu/mole sovalues that the anisotropy is slightly reduced compared with that at room temperature. After a slight decrease of both x~ and y~below 95 K, there follows a paramagnetic upturn which is however stronger for x~than for yj. Assuming paramagnetic contribution from 2~(3d3),athe measured susceptibilities can beinterlayer written as V a sum of an almost temperature-independent intrinsic susceptibility y~and a paramagnetic Curie term y,~= CIT (S = 3/2 and C = 1.875 for spin-only V24): Xmoie = ~ + y.,,,. The solid lines in Fig. 1 represent the data as measured, the broken lines are obtained after substraction of the isotropic susceptibility 23 at.°/~ (inter2~.A similar of substraction of imlayer) paramagnetic V purity effects is possible with a temperature-dependent C, based on an impurity Vieck-type 3~(3d2)wofith2.3at.°/®Van spin-orbit coupling contriparamagnetic V buting to C7’8 This case would imply a relatively large contribution of about 25% and 15% in the experimental values at 300 and 1000 K resp. The corrected susceptibiities (broken lines) give a better image of the intrinsic Pauliparamagnetic term ~ which varies strongly below 100 K, and of the increased low-temperature

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ELECTRICAL PROPERTIES OF VSe2 SINGLE CRYSTALS

Vol. 20, No.3

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Fig. 1. Molar magnetic susceptibilities of VSe2 single crystals (composition V101s Se2) from S to 300 K for an applied magnetic fieldH= 12.40 kOe parallel (xii) and perpendicular (y) to the trigonal c-axis of Cd(OH)2 type VSe2. The solid lines represent the data as measured, the 2~(d); broken seelines text.have been corrected for paramagnetic V

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Fig. 4. Temperature dependence of the in-plane resistivity (p) and Hall coefficient (RH) of a VSe 2 single-crystal plate (composition V101s Se2). Electrical current

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parallel, magnetic field perpendicular to the basal plane.

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Fig. 2. Molar magnetisation measurements of VSe2 single crystals (composition V101s Se2) at 70K as a function of magnetic field (H) for H parallel (M11) and perpendicular (M1) to the trigonal c-axis.

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uK) Fig. 3. Molar magnetic susceptibility randomly oriented powdered VSe2 (composition V1 ~s Se2) from 2 to 1000 K for an applied magnetic field H = 8.75 kOe.

anisotropy. This susceptibility anisotropy, which remains after substraction of an isotropic paramagnetic 2~(d),could contribution of only 2.3 extra at.°/~ interlayer indicate that part of the interlayer VVis present as V3~(d2)with preferential orientation of its magnetic moments in the hexagonal layers due to single-ion anisotropy (contributions of spin—orbit coupling and lowsymmetry trigonal D~ crystal field8). Magnetisation measurements at 70K in fields up to 4OkOe (Fig. 2)

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Fig. 5. Temperature dependence of the measured in-plane resistivity (p. solid line), the calculated impurity resistivity (ps, broken line), and the remaining lattice resistivity (p p1, dash—dotted line) of the same VSe2 crystal as in Fig. 4. give a field-independent behaviour for Xii; ~ is always a little lower than Xii, but approaches the value of Xii for fields above 30 kOe. The high-temperature susceptibility was studied on randomly oriented powdered VSe 2 with composition V1~Se2(Fig. 3). The average molar susceptibility is a —

Vol. 20, No.3

ELECTRICAL PROPERTIES OF VSe2 SINGLE CRYSTALS

sum of the parallel perpendicular susceptibilities, 3XI( +and 2/3)(j. Xmoie gradually decreases i.e. Xmo~ = l/ with increasing temperature, shows a small anomaly around 500 K and reaches a constant value of 190 x 10-6 emu/mole above 900 K. Its low-temperature behaviour gives a somewhat better resolved anomaly at 95 K than the single crystals and shows a smaller paramagnetic upturn, which is consistent with the nearly ideal stoichiometry of this powder. Measurements of the resistivity and Hall effect were made between 2 and 300 K with a d.c. four-probe method, using silver paint for electrical contacts at the edges of the crystal plate. In the resistivity measurements a constant current of 1 mA was applied in the basal plane (Ic-axis); the in-plane Hall current was 100 mA and the applied magnetic field H perpendicular to the basal plane (lie-axis) was 14.95 kOe. During the Hall measurements the magnetic field was commutated several times to eliminate any parasitic thermo voltage over the Hall contacts after each electrical zeroing with H = 0. The Hall voltage could be detected with an accuracy of ±0.02 pV. The crystal plate, cut, in a rectangular shape with 3,was dimensions 8.1 x substrate 3.3 x 0.030 mm mounted onofanabout insulating with a photoprinted copper circuit to which shielded copper wires were soldered to make the electrical connections to the external equipment. Figure 4 shows the temperature dependence of the in-plane resistivity (p)and the Hall coefficient (RH). RH is n-type over the whole temperature range and decreases from a residual value of 308 x l0~cm3 /Coulomb at very low temperatures to a 18 times smaller value at room temperature. No sign reversal ofRH was observed below 300 K. p shows a positive temperature dependence and increases from a residual value of 58pf1.cm at very low temperatures to a 5.75 times larger value at room temperature. Both RH and p have strong anomalies around 100 K, the same temperature where the Pauliparamagnetic susceptibility

253

remaining partofp charge p1 ofcarriers the resistivity willvibrations be due to the scattering by lattice (Fig. 5). The lattice mobifity pj R~(p psi’ decreases strongly with increasing temperature. However, in the curve of p~’vs T no strong anomaly occurs at 100 K. This indicates that the anomalous behaviour of the electrical properties below 100 K is due mainly to an anomalous temperature dependence of the charge-carrier concentration. For a free-electron model the charge-carrier concentration can be calculated directly from the Hall effect; at 2, 100 and 300K one obtains in this way values of 0.12, 0.48 and 2.2 electrons per V atom, respectively. For non-spherical Fermi surfaces the relation between the Hall constant and the carrier concentration is more complicated,9 but the calculated carrier concentration is still of the same order of magnitude. Therefore, the observed high carrier concentration at 300 K is cornpatible with a model in which at 300 K all d electrons (one per V atom) contribute to the metallic conduction. The Hall data (Fig. 4) show that the effective carrier concentration strongly decreases below 100 K. Electron-diffraction patterns of VSe 2 of recorded 300, 140 and 40K indicate the formation a CDW,at which in turn introduces a periodic lattice distortion (PLD).4 This PLD appears to be incommensurate for temperatures above 100 K, but commensurate at lower temperatures and to define a 4a x 4a x 3c superlattice. It is well-known that a CDW is related to distortions of the energy bands near the Fermi surface, which lead to a decrease of the density of states at the Fermi surface.1°A decrease of the density of states corresponds to a decrease of the effective carrier concentration, describing Hall effect and conductivity data. Therefore, the observed decrease of the effective carrier concentration below lOOK can be attributed to the CDW distortion of the crystal. A CDW also leads to anomalies of the magnetic susceptibility of the type as observed for VSe 2 .~o —

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varies strongly. The residual resistivity can be ascribed to the scattering of charge carriers by interlayer vanadium atoms. This contnbution p,(T) to the resistivity is proportional to the charge-carrier concentration to the power 2/3, i.e. one expects p1(T) = p,(O) ~ [RH(T)/RH(0)] 2/3 The —

Acknowledgements The authors wish to thank Miss R. Haange andinMrs. Druiven J. Slijkhuis of forsingletheir contributions the H. growth andand measurements crystal samples of VSe2,and Mr. A. Meetsma for doing the chemical analyses. —

REFERENCES 1. 2.

BLOEMBERGEN J.R., HAANGE R., WIEGERS G.A. & VAN BRUGGEN C.F, VInt. Conf on Solid Compounds of Transition Elements, Uppsala, Sweden, 20—25 June 1976 (to be published). ROST E. & GJERTSEN L., 1 Anorg. Alig. Chem. 328, 299 (1964).

3.

THOMPSON A.H. & SILBERNAGEL B.G., Bull. Am. Phys. Soc. 21,260(1976).

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WILLIAMS P.M., in, Physics and Chemistry ofMaterials with Layered Structures (Edited by LEVY F.), Vol. 2, p. 51. D. Reidel, Dordrecht, Holland (1976).

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VAN BRUGGEN C.F., Thesis, University of Groningen, The Netherlands (1969). FIGGIS B.N., LEWIS J., MABBS F.E. & WEBB G.A., J. Chem. Soc. (A), 1411(1966).

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