Twinning in Incommensurate and Commensurate Structures of ReSi(sub 2-x) Alloys

摘要

ReSi(sub 2-x) is usually referred to as rhenium disilicide but it actually has a211u001estoichiometry of ReSi(sub 1.75). Also it is usually described as having the 211u001etetragonal MoSi(sub 2)C11(sub b) structure but in fact it exhibits a number of 211u001eincommensurate and commensurate structures which are based on the C11(sub b) 211u001estructure. Structure A has an incommensurate periodicity of 4.14a along the a 211u001eaxis. It has a small orthorhombic distortion (b/a(approximately)1.005) 211u001eaccompanied by twinning on the (110) plane and a small monoclinic distortion (b 211u001eapproximately 90.2 degree) accompanied by twinning on the (001) plane. Structure 211u001eB is also incommensurate but has a larger monoclinic angle of 107 (degree); B 211u001eappears to result from a shear transformation of A, giving rise to (001) twins. 211u001eProlonged annealing at high temperatures results in structure C which is 211u001ecommensurate and has planes parallel to, and exactly 4 times the spacing of, the 211u001e(101) planes of the underlying C11(sub b) structure. Structure C is also twinned. 211u001eThe various structures are apparently caused by attempts to accommodate the Si 211u001estructural vacancies. The relationship between these structures and the Nowotny 211u001echimney-ladder structures that have the general formula T(sub n)Si(sub 2n-m) and 211u001eare based on the TiSi(sub 2) structure is discussed.