Abstract: A method of growing a single crystal of KTiOPO.sub.4 which is a nonlinear optical material is disclosed. Growth of the single crystal of KTiOPO.sub.4 is carried out by melting a KTiOPO.sub.4 material with a flux to produce a melt, then contacting a seed crystal to the melt, and by slowly cooling at a saturation temperature or below. At this time, molar fractions of K.sub.2 O, P.sub.2 O.sub.5 and TiO.sub.2 contained in the melt fall within a region surrounded by six point in a K.sub.2 O-P.sub.2 O.sub.5 -TiO.sub.2 ternary phase diagram of A (K.sub.2 O:0.4150, P.sub.2 O.sub.5 :0.3906, TiO.sub.2 : 0.1944), B (K.sub.2 O:0.3750, P.sub.2 O.sub.5 : 0.3565, TiO.sub.2 : 0.2685), C (K.sub.2 O: 0.3750, P.sub.2 O.sub.5 : 0.3438, TiO.sub.2 : 0.2813), D (K.sub.2 O: 0.3850, P.sub.2 O.sub.5 : 0.3260, TiO.sub.2 : 0.2890), E (K.sub.2 O: 0.4000, P.sub.2 O.sub.5 : 0.3344, TiO.sub.2 : 0.2656), and F (K.sub.2 O: 0.4158, P.sub.2 O.sub.5 : 0.3744, TiO.sub.2 : 0.2098). In addition, K.sub.15 P.sub.13 O.sub.

Abstract: Disclosed is a process for preparing a single crystal of potassium titanium arsenate by flux growth method, wherein a compound of K.sub.2 ZO.sub.4, in which Z is at least one of W, S, Mo and Cr, is used as a flux and the flux is blended with potassium titanium arsenate in such a way that the proportion of potassium titanium arsenate is from 15 to 50 mol % of the whole. The process may be carried out by conventional flux growth method under an atmospheric pressure and gives a single crystal of potassium titanium arsenate at a low cost.