The phase transitions in PbNb 2 O 6 and in compositions of the type Pb 1+x (B x Nb 1‐x )O 6 , where B = Ti 4+ , Zr 4+ , or Sn 4+ , have been investigated between 25° and 650°C. using X‐ray and dilatometric techniques. The modified PbNb 2 o 6 compositions possess orthorhombic PbNb 2 O 6 ‐type structure, with the additional Pb 2+ ions occupying vacant lattice sites. The lattice parameters a and c expand and b contracts during heating until, at the ferroelectric Curie temperature, a and b suddenly coincide and c expands slightly. Besides this phase change at the Curie temperature, the nonstoichiometric compounds show an additional phase transition in the range 450° to 300°C. depending on composition. The intermediate phase of Pb l+x (Ti 2 Nb 1‐z ) 2 O 6 appears to possess orthorhombic symmetry.
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E. C. Subbarao (1960) studied this question.
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