  We present a detailed numerical investigation of five nonlinear materials and
their properties regarding photon-pair creation using parametric
downconversion. Periodic poling of ferroelectric nonlinear materials is a
convenient way to generate collinearly propagating photon pairs. Most
applications and experiments use the well-known potassium titanyl phosphate
(KTiOPO4, ppKTP) and lithium niobate (LiNbO3, ppLN) crystals for this purpose.
In this article we provide a profound discussion on the family of
KTP-isomorphic nonlinear materials, including KTP itself but also the much less
common CTA (CsTiOAsO4), KTA (KTiOAsO4), RTA (RbTiOAsO4) and RTP (RbTiOPO4). We
discuss in which way these crystals can be used for creation of spectrally pure
downconversion states and generation of crystal-intrinsic polarisation- and
frequency entanglement. The investigation of the new materials disclosed a
whole new range of promising experimental setups, in some cases even
outperforming the established materials ppLN and ppKTP.
