We successfully grew langbeinite-type structures containing Ti 4+ and Ln 3+ ions as M 2 Ti 2-x Ln x (PO 4 ) 3 crystals (M = K + or Rb + and Ln = Er 3+ or Yb 3+ ) by high-temperature-solution methods. We took accurate composition measurements of these crystals, paying special attention to the lanthanide contents. We resolved the structure of the Rb 2 Ti 1.01 Er 0.99 (PO 4 ) 3 crystal and studied the evolution of this structure with the temperature. We found that this compound decomposed at 1323 K. We made a preliminary study to check the nonlinear optical properties of these materials, showing that the second-harmonic generation efficiency of the Rb 2 Ti 1.01 Er 0.99 (PO 4 ) 3 was at least similar to that of potassium dihydrogen phosphate.