PHYSICAL REVIE%' B VOLUME 39, NUMBER l7 15 JUNE 1989-I Electronic instabilities of the quasi-two-dimensional monophosphate tungsten bronze P4%12044 Enoch Wang and Martha Greenblatt Department of Chemistry, Rutgers-The State University of New Jersey, ¹w Brunswick, New Jersey 08903 Idris El-Idrissi Rachidi and Enric Canadell Laboratoire de Chimie Theorique, Universite de Paris-Sud, 91405 Orsay, France Myung-Hwan Whangbo Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 S. Vadlamannati Department of Physics, Rutgers-The State University of New Jersey, ¹w Brunswick, ¹w Jersey 08903 (Received 29 December 1988) We report results of electrical resistivity, Hall eA'ects, and magnetic susceptibility measure- ments on oriented single crystals of P4%'l2044, which suggest that P4WI2044 has two resistivity anomalies arising from charge-density-wave instabilities. Tight-binding electronic-band-structure calculations on the %602' slab of P4WI2044 support the above suggestion. INTRODUCTION Low-dimensional metals are interesting because of their physical properties associated with electronic instabilities such as charge-density waves (CDW) or spin-density waves. Several quasi-two-dimensional (2D) molybdenum bronzes, Nao9Mo60~7, ' Ko9Mo60~7, ' and TIMo60t7, as well as the Magneli phases y- and rl-Mo40/ / ' have been shown to exhibit CDW instabilities. P4W~z044 (Ref. 5) is isostructural with y-Mo40~ ~, and belongs to the family of the monophosphate tungsten bronzes with pentagonal tun- nels (MPTBt, ), (PQi)4(WO3)2~. Although P4Wt2044 and y-Mo40~ ~ are isostructural, they are not isoelectronic. There are two electrons to fill the bottom three d-block bands of P4W~2044, but four electrons to fill the corre- sponding band of y-Mo40~t. As schematically shown in Fig. 1, P4W~2044 contains Re03-type slabs of corner- '&%i b gI L FIG. 1. Structure of the monophosphate tungsten bronze with pentagonal tunnels, MPTBt, [(PO2)4(WO3)2~, m 6] pro- jected along the a crystallographic direction. sharing W06 octahedra (with the formula W6022), which terminate with P04 tetrahedra. Empty pentagonal tun- nels are created at the junction between the Re03-type slabs and the P04 tetrahedra. In the present paper, we re- port results of electrical resistivity, Hall effects, and mag- netic susceptibility measurements on oriented single crys- tals of P4%~2044, which suggest that P4W~2044 has two resistivity anomalies arising from CDW instabilities. We interpret these experimental observations on the basis of the tight-binding electronic band structure calculated for a W60q2 slab. EXPERIMENT Polycrystalline P4W~2044 was prepared according to a previously reported method. A stoichiometric mixture of (NH4)2HP04 and W03 was first heated at 650 C to decompose the ammonium hydrogen phosphate. Ap- propriate amounts of W metal was then added to the decomposed product and the mixture was pelletized and sealed in an evacuated quartz tube for reaction at — 1000'C for at least four days. Single crystals of P4%~2044 used in the characterization of electronic prop- erties were obtained from an initial composition of P6W)p044. The reaction for P6Wlp044 was carried out at 1150'C for at least five days before annealing to room temperature. The cooling rate was increased from 0.6'C/h to 2'C/h as the temperature was lowered. P4W&2044 crystals were separated from the matrix mechanically and washed in hot concentrated HF before attaching electrical contacts. Low-temperature (2-300 K) resistivity was measured using a standard, four-probe technique with ultrasonically soldered indium contacts on single crystals. The largest error in the measurement is about 25%. Magnetic suscep- tibility was measured in a Quantum Design superconduct- ing quantum interference device magnetometer from 3 to 12969 @1989The American Physical Society