Structure of molten GeSe by neutron diraction: the Ge coordination environment I. Petri a , P.S. Salmon a, * , H.E. Fischer b a Department of Physics, University of Bath, Bath BA2 7AY, UK b Institut Laue-Langevin, Avenue des Martyrs, F-38042 Grenoble cedex, France Abstract The structures of liquid and solid GeSe are studied by using neutron diraction. The results for the solid con®rm that it undergoes a phase transition from a distorted NaCl-type structure at low temperatures to a regular NaCl-type structure prior to melting. Application of the method of isotopic substitution in neutron diraction to the liquid at 727(2)°C has allowed the coordination environment of Ge to be investigated. The results are consistent with Ge being essentially fourfold coordinated to 3 Se and 0.8(2) Ge. Ó 1999 Elsevier Science B.V. All rights reserved. 1. Introduction The object of this paper is to present new results on the coordination environment of Ge in molten GeSe as measured by using the method of isotopic substitution in neutron diraction. Motivation for this work stems in part from a previous neutron diraction study on the liquid at the total structure factor level which indicates that the local ordering in the liquid more closely resembles that of the low-temperature (LT) rather than the high-tem- perature (HT) phase of crystalline GeSe [1]. LT- GeSe has a distorted NaCl-type structure in which each chemical species makes six bonds, three weaker than the other three, to unlike chemical species to form a double-layer structure [2±5]. It is reported to undergo a structural phase transition to a normal NaCl-type structure at 651(5)°C [6] before melting incongruently at 675(2)°C [7]. However, the neutron diraction experiment on the liquid gave a coordination number of less than six i.e. of 3.6(3) Se around Ge on the assumption that there is no homo-polar bonding [1]. Information on liquid Ge x Se 1x (0 6 x 6 1) compounds at the partial structure factor level is also required to help explain a change in the nature of the bonding from metallic to covalent as x is decreased from unity [1,8,9]. Molten GeSe is a semiconductor with an electrical conductivity r 40 X 1 cm 1 just above its melting point and positive temperature coecient dr=dT [10,11]. Thermopower measurements are consistent with p-type conductivity [11] while the sign of the Hall coecient is reported to change from positive to negative on melting [12]. An interpretation of the Hall coecient in disordered materials is however problematic [13]. 2. Theory In a neutron diraction study of GeSe, the co- herent scattered intensity can be represented by the total structure factor Journal of Non-Crystalline Solids 250±252 (1999) 405±409 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: 44 1225 323 698; fax: 44 1225 826 110; e-mail: p.s.salmon@bath.ac.uk 0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 0 9 - 9