Experimental evidence of the pyroelectric nature of struvite

Struvite (MgNH₄PO₄·6H₂O) is a mineral first identified in 1845. It is being tested for several reasons: (1) it is a problem in liquid wastewater treatment plants; (2) on the other hand, it is recovered from these wastewaters for its phosphorus, magnesium and nitrogen; (3) is the main component of microbial-induced urinary stones. We have recently shown that struvite is ferroelectric and piezoelectric. In this paper, we present the first experimental evidence of the pyroelectric nature of struvite. Using a single-diffusion gel growth technique, we grew struvite crystals as flat, parallel plates. We performed measurements of pyroelectric currents on struvite of this shape, using it as a dielectric of a plate capacitor. The occurrence of pyroelectric effects in struvite was investigated by measuring depolarization currents as a function of temperature. This technique allows the disclosure of ferroelectric/pyroelectric transitions as well as the reconstruction of the ferroelectric loop. We found that the value of the pyroelectric coefficient p attains a maximum of 22 × 10⁻⁶ C m⁻² K⁻¹ slightly below room temperature, going down to p ≅ 10 × 10⁻⁶ C m⁻² K⁻¹ at room temperature. This value is comparable to values for other minerals. For example, the pyroelectric polarization coefficient of tourmaline, the mineral for which the pyroelectric effect was first discovered, is 4 × 10⁻⁶ C m⁻² K⁻¹. This value is 2.5 times lower than that measured for struvite, illustrating struvite's fairly strong pyroelectricity.