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Mixed crystals Pb3(PO4)2--Pb3(AsO4)2 are ferroelastic at low temperatures. The phase diagram contains three different phases a (paraelastic), 
and 
(both ferroelastic). The transformation 
- is first order for Pb3(PO4)2 and approaches second-order behaviour with increasing As content. The spontaneous strains and the birefringence follow approximately a Curie-Weiss law, with a corresponding order-parameter exponent β = ¼. The mutual high-temperature phases give diffuse X-ray reflections due to inelastic scattering. The soft-mode model is discussed for these results.
and 
(both ferroelastic). The transformation 
- is first order for Pb3(PO4)2 and approaches second-order behaviour with increasing As content. The spontaneous strains and the birefringence follow approximately a Curie-Weiss law, with a corresponding order-parameter exponent β = ¼. The mutual high-temperature phases give diffuse X-ray reflections due to inelastic scattering. The soft-mode model is discussed for these results.
