The laser and phosphor host material with nominal composition LaMgAl11O19 is shown to be nonstoichiometric. Czochralski grown crystals are hexagonal, with space group P63/mmc, and a=5.589 82(5), c=21.9738(12) Å at 295 K. The thermal expansivity has been determined over the range 295 to 860 K: at 295 K, α1=4.97(2)×106 K1 and α3=8.14(3)×106 K1and at 775 K, α1=7.01(1)×106 K1 and α3=10.1(2)×106 K1. The integrated intensities of 20 487 reflections within a reciprocal lattice sphere of radius (sin θ)/λ=1.0 Å1 were measured on a CAD‐4 diffractometer with MoKα radiation, resulting in 18 232 reflections above background. Correction and averaging gave 864 independent structure factors with an internal agreement factor of 0.0145. The crystal structure was solved from the Patterson function and Fourier series and refined by the method of least squares. The final agreement factor R=0.013 98. The composition corresponds to La1−xMgAl11+xO19, with x=0.0502(8): the La deficiency is exactly compensated by excess Al. The principal La site is not fully occupied, but contains 4.76 at. % Al and 5.2 at. % Schottky defects. The remaining 4.94 at. % La and 0.26 at. % Al are distributed over two threefold satellite sites 0.583(3) Å distant from the principal La sites. The Mg atoms share a tetrahedral site equally with Al, while the Al atoms in the trigonal bipyramidal site undergo anharmonic motion. The remaining Al atoms occupy distorted octahedral or trigonal antiprismatic sites, with the latter sharing faces to give an Al–Al distance of 2.6541(9) Å. Other polyhedra share only oxygen atom corners or edges. The superior lasing characteristics are due at least partially to the wide separation of 5.5898 Å between principal La sites. The small average atomic radial vibrational amplitude of 0.071 Å corresponds to a Debye characteristic temperature of 631 K.

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