Abstract: Mixed strontium and lanthanide oxides and a laser using monocrystals of these oxides.These oxides with a magnetolead type crystalline structure have the following formula:Sr.sub.x Ln1.sub.y1 Ln2.sub.y2 Ln3.sub.y3 M.sub.z A.sub.a B.sub.b O.sub.19--kin which Ln1 represents at least one trivalent element selected from lanthane, gadolinium and yttrium; Ln2 represents at least one trivalent element selected from neodymium, praseodymium, erbium, holmium and thulium; Ln3 represents an element selected from bivalent europium or trivalent cerium with retention of electric neutrality by virtue of oxygen holes; M represents at least one bivalent metal selected from magnesium, manganese and zinc; A represents at least one trivalent metal selected from aluminum and gallium; B represents a trivalent transition metal selected from chromium and titanium; x, yl, y2, y3, z, a, b and k represent numbers so that 0<x<1, 0.ltoreq.y1<1, 0<y2<1, 0.ltoreq.y3<1, 0<z<1, 10.5<a<12, 0.ltoreq.b.ltoreq.0.

Abstract: A mixed aluminum oxide having the following formula:X.sup.1.sub.(x.sbsb.1.sub.-x.sbsb.2.sub.) X.sup.2.sub.(x.sbsb.2.sub.) M.sup.1.sub.(y.sbsb.1.sub.-y.sbsb.2.sub.) M.sup.2.sub.(y.sbsb.2.sub.) Al.sub.(z.sbsb.1.sub.-z.sbsb.2.sub.) M.sup.3.sub.(z.sbsb.2.sub.) O.sub.19wherein X.sup.1 and X.sup.2, which can be the same or different, represent a metal selected from the group consisting of lanthanum, praseodymium, neodymium, samarium and gadolinium, provided that X.sup.1 and X.sup.2 are different when X.sup.1 or X.sup.2 represents lanthanum or gadolinium; M.sup.1 and M.sup.2, which can be the same or different, each represent a metal selected from the group consisting of magnesium and divalent transition metals; M.sup.3 represents a trivalent transition metal; x.sub.1 represents a number from 0.8 to 1; y.sub.1 represents a number from 0.7 to 1; z.sub.1 represents a number from 10 to 12; x.sub.2 represents a number from 0 to x.sub.1 ; y.sub.2 represents a number from 0 to y.sub.1 ; and z.sub.