Optical and x-ray studies of thin mixed crystals of NaCl-typed RbI and CsCl-typed CsI

Thin Rb1−xCsxI mixed crystals have been grown from the melt in a very small gap formed in between a pair of quartz plates and reflection spectra were measured in the uv-light region at low temperature. Continuous spectral changes of the exciton bands with the composition x suggest strongly the formation of extended solid solution both in the RbI-rich (0.0⩽x⩽0.4) and CsI-rich (0.7⩽x⩽1.0) regions. The formation was confirmed by x-ray diffraction, and the compositional change of the lattice constant was determined. These results are in contrast to the eutectic type phase relation with only a limited solid solubility observed in the bulk state of the RbI–CsI binary system.