Degenerate four-wave mixing spectroscopy is used to measure the diffusion of electronic excitation in the system Nd
xLa
(1-x)P
5O
14. For samples with

, 0.5, and 1.0, diffusion lengths of 165, 242, and 253 nm were obtained. When the period of the grating is less than 1.3μm and greater than 2.2μm, the observed scattered beam decay exhibits unusual features which are discussed in terms of excitation trapping and the influence of other types of laser-induced grating phenomena.