Low-temperature polarity in antiferroelectric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>PbZrO</mml:mi> <mml:mn>3</mml:mn> </mml:msub> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/199

At low temperatures, additional phase transitions have been detected in the antiferroelectric crystals <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>PbZrO</a:mi> <a:mn>3</a:mn> </a:msub> </a:mrow> </a:math> and <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:msub> <b:mi>PbHfO</b:mi> <b:mn>3</b:mn> </b:msub> </b:mrow> </b:math> near 270 K. In this paper we investigate these transitions theoretically and experimentally and arrive at the conclusion that it is of first order, using the free energy expanded to a sixth order in the order parameter. In addition to recently published pyroelectric and dielectric properties, the presence of this transition has been proven by measuring the temperature dependence of the specific heat and natural absorbance, and compared with the literature data of the structural investigations by neutron scattering. Opposite to the previous articles, the origin of this transition is not connected with the Pb ion displacements, but it is solely caused by the p-d interaction in the transition metal-oxides <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:msub> <c:mi>ZrO</c:mi> <c:mn>6</c:mn> </c:msub> </c:mrow> </c:math> and <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:msub> <d:mi>HfO</d:mi> <d:mn>6</d:mn> </d:msub> </d:mrow> </d:math> substructures, typical for a broad number of ferroelectric oxide perovskites.