Phase transitions in binary intermetallic compounds with the CsCl-type structure have been studied by a novel combination of high-temperature powder X-ray diffraction and Rietveld full-profile refinement. The Landau theory of symmetry and phase transitions and the Gibbs-Knonvalow equation have been applied to understand the phase behavior of some systems with the CsCl-type structure;Alloys in near equiatomic MnAu have the CsCl-type structure with Pm3m symmetry at high temperature. With decreasing temperature the cubic phase distorts to the AuCu-type tetragonal phase with P4/mmm symmetry with c/a 1;The nonstoichiometric compounds RhTi, NbRu, and RuTa with the CsCl-type structure at high temperature undergo thermal symmetry breaking transitions upon cooling. The transitions are first to the AuCu-type tetragonal, and then to the orthorhombic NbRu-type with Cmmm symmetry. Alloys Ir-Ti which are titanium rich have the CsCl-type structure. This cubic structure transforms to the AuCu-type tetragonal structure and then to the NbRu-type structure with increasing atomic percent iridium;The high-temperature form of nonstoichiometric VIr has the AuCu-type structure, and the low-temperature form is the VIr-type orthorhombic structure with Cmmm symmetry. The lattice parameters for the orthorhombic structure are a = 2a[subscript] tet, b = 2b[subscript] tet, and c = a[subscript] tet. The phase transition appears to occur continuously;New partial phase diagrams for the composition ranges in near equiatomic MnAu, NbRu, and RuTa are also presented
