Monocrystalline substrates of gadolinium yttrium penta- ultraphosphate are coated with an epitaxial layer of neodymium yttrium penta- ultraphosphate to produce a wave guiding, laser active, solid state devices. The preparation procedure involves heating a solution of neodymium oxide and yttrium oxide in phosphoric acid to about 500 C. with holding points being interposed at about 180 C. and at about 370 C. After the solution has been held at 500 C. for about 12 through 15 hours, a preformed substrate monocrystal of gadolinium yttrium penta-ultraphosphate is introduced thereinto and a layer of monocrystalline neodymium yttrium penta-ultraphosphate is grown thereon.
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