I. Němec, I. Matulková, W. Krumbe, L. Andersen, I. Císařová, J. Kroupa, P. Němec, L. Bohatý, and P. Becker. Linear and nonlinear optical properties, pyroelectricity and vibrational spectroscopy of polar guanidinium hydrogen phosphite, {GuH}2PO3, and hydrogen selenite, {GuHSeO}3. Optical Materials, Elsevier BV, volume 111, January 2021.