Crystalline material, method of making and using the same

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The present invention relates to a crystalline material comprising a basic crystal based on at least one host element (We), which basic crystal is doped with at least one element of the rare earths (Do), the crystalline material having the formula : €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ We1 (a) We2 (b) Ta (c) Do1 (d) Do2 (e) Hal (w) O (x) Te (y) Se (z) , where We1 and We2 are independently and differently selected from elements from the group consisting of niobium, aluminium, scandium, barium, gadolinium, lanthanum, yttrium, cerium, calcium and titanium; Do 1 and Do 2 are independently and differently selected from elements of the group of rare earths; Hal is fluoride, chloride, bromide or iodide; the indices (a)-(z) each being 0 to 3 independently of one another, though at least one of (a) and (b) is greater than 0, at least one of (d) and (e) is greater than 0, and at least one of (w), (x), (y) and (z) is greater than 0; wherein the sum of (a)+(b)+(c)=1-2, preferably 1 or 2, and the sum of (w)+(x)+(y)+(z)=2-3, preferably 2 or 3, with the proviso that (c) is > 0 if - We 1 = Ca, We 2 = Sc, Do 1 = Yb or Tm and (e)=(w)=(y)=(z)=0, or - We 1 = Ce, Do 1 = Eu or Er and (b) = (e) = (w) = (y) = (z) = 0, wherein (x) is >0.