. . "where L is an element selected from the group consisting of Europium (Eu), Strontium (Sr), Barium (Ba), or Calcium (Ca); M is a metal selected from Group IV (preferably Pb, Sn or Ge), D is a non-metal selected from Group VI (preferably Te, Se or S), J is a non-metal selected from Group VI (preferably Te, Se or S), R is a Group V material (preferably Sb, As or P); and where 0???x???1, 0???y???1 and" . .