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High pressure electrical resistivity of HgCr2Se4
Journal of the Less-Common Metals, 98 (1984)
L13 - L15
L13
Letter
High pressure electrical
resistivity
P. KISTAIAH, K. SATYANARAYANA Department
of HgCr2Se4 MURTHY and K. V. KRISHNA RAO
of Physics, Osmania University, Hyderabad 500007
(India)
(Received December 6, 1983)
The ternary chromium chalcogenide compound HgCr,Se4 is a ferromagnetic semiconductor and crystallizes in the cubic spine1 structure. Most interest in this compound centres on its typical properties arising from the interaction of localized Cr3+ d electrons and conduction electrons (or holes). The existence of localized and conduction electrons in the same material gives a strong dependence of the transport properties on the magnetic order, the giant magnetoresistance, the spontaneous Hall effect and the anisotropic resistivity. Although many properties of interest to semiconductor physics have been measured on HgCr,Se, at ambient pressure [ 1 - 31, no results on the pressure dependence of its electrical resistivity have yet been reported. In this letter, we report the study of the electrical resistivity of HgCr,Se, at pressures up to 70 kbar as part of a prog-ramme of high pressure conductivity studies on some semiconducting compounds. Single crystals of HgCr,Se4 were grown by the chemical transport method with the use of CrCl, as a transport agent. The material was very crystalline and its X-ray powder pattern confirmed the spine1 structure (space group, Fd3m) with a lattice constant acubic = 1.0665 + 0.0004 nm at ambient temperature and pressure conditions. The details of the growth and purity analysis of HgCr,Se, have been described elsewhere [ 41. The high pressure experiments were carried out in a high pressure cell, consisting of two Bridgman anvils of tip diameter 4 mm. The sample (about 1.5 mm in length and 0.1 mm2 in cross section) whose electrical resistivity was to be measured was placed on the anvil tip and embedded in a steatite pressure-transmitting medium and surrounded by a heat-treated pyrophyllite gasket. A mixture of ferric oxide and epoxy is used for binding the copper electrical leads and pyrophyllite disc. The cell was pressurized in a hydraulic press to any desired pressure up to 70 kbar. The cell was calibrated against the polymorphic phase transitions of bismuth and ytterbium. The details of the high pressure cell assembly and calibration have been described elsewhere [ 51. Four independent high pressure runs were made on HgCr2Se4. The normalized resistivity determined from the two-probe and four-probe techniques as a function of pressure was found to be reproducible. A plot @ Elsevier Sequoia/Printed
in The Netherlands
Pressure
Fig. 1. Normalized
resistivity
[k
bar]
us. pressure
curve for HgCr#ed
at 300 K.
of the normalized resistivity uersus the sample pressure (Fig. 1) shows some interesting features. The resistivity decreases by more than one order of magnitude in the pressure range from about 5 to 15 kbar. The normalized resistivity increases by almost 8% when the pressure is increased to 17 kbar. After this, the resistivity remains constant with further increases in pressure. At about 60 kbar, it drops once again by almost 5%. The decrease of more than one order of magnitude in the resistivity of the semiconducting compound HgCr,Se, in the 5 - 15 kbar pressure interval signals a transition of its semiconducting state to a metallic state. This behaviour is similar to that reported by Joshi et al. [6] and Kistaiah et al. [7] for CrSe and GaSe respectively at elevated pressures and at ambient temperature. The high pressure resistivity behaviour of HgCr$e, in the pressure range 15 - 70 kbar is analogous to that observed in metallic bismuth [ 81. Recently we have studied the high temperature lattice thermal behaviour of HgCr,Se, at elevated temperatures up to about 850 K. It was found that at around 650 K this compound undergoes a structural phase transition from its cubic spine1 structure to a structure of lower symmetry. The resistivity anomaly of this compound at 17 kbar may be connected with this structural transition. However, for a complete understanding of this interesting high pressure resistivity behaviour of HgCr2Se4, it is necessary to carry
L15
out high pressure and high temperature X-ray studies on this compound. We are working on this aspect and the detailed results will be published shortly.
The authors are grateful to Professor P. Gibart of Laboratoire de Magnetisme, Centre National de la Recherche Scientifique, for supplying the compound and for many fruitful discussions. They are also grateful to Professor E. S. Rajagopal of the Indian Institute of Science, Bangalore, for providing laboratory facilities. The experimental assistance of Dr. A. K. Bandyopadhyay is also acknowledged.
1 L. Goldstein, P. Gibart and A. Selmi, J. Appl. Phys., 49 (1978) 1474. 2 M. N. Iliev, E. Anastassakis and T. Arai, Phys. Status Solidi B, 86 (1978) 717. 3 A. Selmi, P. Gibart, L. Goldstein and A. Tomas, Jpn. J. Appl. Phys., Suppl., 19 (3) (1980) 267. 4 P. Gibart, J. Cryst. Growth, 43 (1978) 21. 5 A. K. Bandyopadhyay, A. V. Nalini, E. S. Rajagopal and S. V. Subramanyam, Rev. Sci. Instrum., 51 (1980) 136. 6 D. K. Joshi, C. Karunakaran, S. N. Vaidya and M. D. Karkhanvala, Mater. Res. Bull., 12 (1977) 1111. 7 P. Kistaiah, T. R. Prasad, K. S. Murthy, L. Iyengar and K. V. K. Rao, N&Z. Acad. Sci. Lett., 3 (1980) 91. 8 P. W. Bridgman, The Physics of High Pressures, Bell, London, 1952.
L13 - L15
L13
Letter
High pressure electrical
resistivity
P. KISTAIAH, K. SATYANARAYANA Department
of HgCr2Se4 MURTHY and K. V. KRISHNA RAO
of Physics, Osmania University, Hyderabad 500007
(India)
(Received December 6, 1983)
The ternary chromium chalcogenide compound HgCr,Se4 is a ferromagnetic semiconductor and crystallizes in the cubic spine1 structure. Most interest in this compound centres on its typical properties arising from the interaction of localized Cr3+ d electrons and conduction electrons (or holes). The existence of localized and conduction electrons in the same material gives a strong dependence of the transport properties on the magnetic order, the giant magnetoresistance, the spontaneous Hall effect and the anisotropic resistivity. Although many properties of interest to semiconductor physics have been measured on HgCr,Se, at ambient pressure [ 1 - 31, no results on the pressure dependence of its electrical resistivity have yet been reported. In this letter, we report the study of the electrical resistivity of HgCr,Se, at pressures up to 70 kbar as part of a prog-ramme of high pressure conductivity studies on some semiconducting compounds. Single crystals of HgCr,Se4 were grown by the chemical transport method with the use of CrCl, as a transport agent. The material was very crystalline and its X-ray powder pattern confirmed the spine1 structure (space group, Fd3m) with a lattice constant acubic = 1.0665 + 0.0004 nm at ambient temperature and pressure conditions. The details of the growth and purity analysis of HgCr,Se, have been described elsewhere [ 41. The high pressure experiments were carried out in a high pressure cell, consisting of two Bridgman anvils of tip diameter 4 mm. The sample (about 1.5 mm in length and 0.1 mm2 in cross section) whose electrical resistivity was to be measured was placed on the anvil tip and embedded in a steatite pressure-transmitting medium and surrounded by a heat-treated pyrophyllite gasket. A mixture of ferric oxide and epoxy is used for binding the copper electrical leads and pyrophyllite disc. The cell was pressurized in a hydraulic press to any desired pressure up to 70 kbar. The cell was calibrated against the polymorphic phase transitions of bismuth and ytterbium. The details of the high pressure cell assembly and calibration have been described elsewhere [ 51. Four independent high pressure runs were made on HgCr2Se4. The normalized resistivity determined from the two-probe and four-probe techniques as a function of pressure was found to be reproducible. A plot @ Elsevier Sequoia/Printed
in The Netherlands
Pressure
Fig. 1. Normalized
resistivity
[k
bar]
us. pressure
curve for HgCr#ed
at 300 K.
of the normalized resistivity uersus the sample pressure (Fig. 1) shows some interesting features. The resistivity decreases by more than one order of magnitude in the pressure range from about 5 to 15 kbar. The normalized resistivity increases by almost 8% when the pressure is increased to 17 kbar. After this, the resistivity remains constant with further increases in pressure. At about 60 kbar, it drops once again by almost 5%. The decrease of more than one order of magnitude in the resistivity of the semiconducting compound HgCr,Se, in the 5 - 15 kbar pressure interval signals a transition of its semiconducting state to a metallic state. This behaviour is similar to that reported by Joshi et al. [6] and Kistaiah et al. [7] for CrSe and GaSe respectively at elevated pressures and at ambient temperature. The high pressure resistivity behaviour of HgCr$e, in the pressure range 15 - 70 kbar is analogous to that observed in metallic bismuth [ 81. Recently we have studied the high temperature lattice thermal behaviour of HgCr,Se, at elevated temperatures up to about 850 K. It was found that at around 650 K this compound undergoes a structural phase transition from its cubic spine1 structure to a structure of lower symmetry. The resistivity anomaly of this compound at 17 kbar may be connected with this structural transition. However, for a complete understanding of this interesting high pressure resistivity behaviour of HgCr2Se4, it is necessary to carry
L15
out high pressure and high temperature X-ray studies on this compound. We are working on this aspect and the detailed results will be published shortly.
The authors are grateful to Professor P. Gibart of Laboratoire de Magnetisme, Centre National de la Recherche Scientifique, for supplying the compound and for many fruitful discussions. They are also grateful to Professor E. S. Rajagopal of the Indian Institute of Science, Bangalore, for providing laboratory facilities. The experimental assistance of Dr. A. K. Bandyopadhyay is also acknowledged.
1 L. Goldstein, P. Gibart and A. Selmi, J. Appl. Phys., 49 (1978) 1474. 2 M. N. Iliev, E. Anastassakis and T. Arai, Phys. Status Solidi B, 86 (1978) 717. 3 A. Selmi, P. Gibart, L. Goldstein and A. Tomas, Jpn. J. Appl. Phys., Suppl., 19 (3) (1980) 267. 4 P. Gibart, J. Cryst. Growth, 43 (1978) 21. 5 A. K. Bandyopadhyay, A. V. Nalini, E. S. Rajagopal and S. V. Subramanyam, Rev. Sci. Instrum., 51 (1980) 136. 6 D. K. Joshi, C. Karunakaran, S. N. Vaidya and M. D. Karkhanvala, Mater. Res. Bull., 12 (1977) 1111. 7 P. Kistaiah, T. R. Prasad, K. S. Murthy, L. Iyengar and K. V. K. Rao, N&Z. Acad. Sci. Lett., 3 (1980) 91. 8 P. W. Bridgman, The Physics of High Pressures, Bell, London, 1952.