A flux process is disclosed for producing a single orthorhombic crystal of Cs.sub.1-x M.sub.x TiOAsO.sub.4 (where M is Na, K, Rb, and/or Tl and x is from 0 to 0.4) wherein the dimension of the crystal along each axis is at least about 2 mm, and wherein the product at the dimensions along the three axes is at least about 15 mm.sup.3. The process involves preparing a homogeneous melt containing the components for forming said crystal and a flux comprising oxides of Cs and As at a temperature no higher than the decomposition temperature of said orthorhombic crystal, the mole fraction of M relative to the total Cs+M in the melt being within the range of from 0 to about 0.2; introducing a seed crystal for said single crystal in the melt; activating the controlled crystallization on the seed crystal; and continuing the crystallization until formation of the single crystal is completed. Single crystals of Cs.sub.1-x M.sub.x TiOAsO.sub.4 (including crystals at least about 5 mm.times.5 mm 5 mm) are also disclosed.