Absolute sense and model for the piezoelectric and pyroelectric coefficients in Ba(NO₂)₂H₂O and Cs₂S₂O₆

The absolute sense of a large single-crystal of Ba(NO₂)₂.H₂O (point group 6), and of one of Cs₂S₂O₆ (point group 6mm), was determined by use of X-ray anomalous scattering methods. The piezoelectric d₃₃ and pyroelectric p₃ coefficients were measured on the same crystals, thereby leading to the assignment of the absolute sense of both coefficients. The magnitudes and senses found for Ba(NO₂)2.H2O at 298 K are p₃ = -14 (1) × 10^-5 C m-2 K^-1 and d₃₃ = +3.1 (2) × 10^-12 C N^-1. The absolute coefficients for Cs₂S₂O₆ at 298 K are p3 = +0.9 (2) × 10^-6 C m^-2 K^-1 and d₃₃ = + 14.7 (1.0) x 10^-12 C N^-1. The atomic displacements in Ba(NO₂)₂.H₂O corresponding to p3 are 5.7 × 10^-4 Å K^-1 by the Ba²⁺ ion or dipolar rotations of 7.7 arc min K^-1 by NO₂ or H₂O: for d₃₃, the displacements are about 0.1 × 10^-4 Å (10⁶ N m^-2)^-1 by Ba²⁺ or dipolar rotations of 0.2 arc min(10⁶ N m^-2)^-1. In the case of Cs₂S₂O₆, the Cs⁺ ion displacements are 0.1 × 10^-4 Å K^-1 on heating and 0.2 × 10^-3 Å (10⁶ N m^-2)^-1 under stress. Dipolar rotations do not contribute either to p3 or to d₃₃ in Cs₂S₂O₆.