Nagelschmidtite as a candidate host phase for actinides, rare earth and different waste elements

Abstract

Nagelschmidtite, Ca7P2Si2O16, is an end-member of continuous solid solution Ca2SiO4 – Ca3(PO4)2⋅2Ca2SiO4 within the pseudo-binary system Ca3(PO4)2 – Ca2SiO4 (whitlockite – larnite). This phase is capable to wide isomorphic exchanges in Ca, P and Si sites: Ca2+ = Sr2+; Ca2+ = Eu2+; Ca2+ + P5+ = (RE,An)3+ + Si4+, 2Ca2+ = Na+ + (RE,An)3+; 2Ca2+ = An4+ + ☐; Ca2+ + Si4+ = (RE,An)3+ + (Al,Fe)3+; Ca2+ + Si4+ = Na+ + P5+; 2Ca2+ = Na+ + (Al,Fe)3+; Ca2+ + P5+ = Na+ + S6+. It was found in metallurgical slags and geological formations. We revealed nagelschmidtite-type phase in vitrified phosphorus-bearing radioactive incinerator slags. The materials were glass-crystalline and contained nano-sized nagelschmidtite crystals distributed in vitreous matrix phase. Average chemical composition of the largest (few microns) crystals was recalculated to formula Na1.21K1.05Ca2.22Al2.02Fe0.46Si2.69P1.26U0.08O15.76. Significant oxygen misbalance suggests higher than U(IV) oxidation state for uranium – U(V) or U(VI). Capability of nagelschmidtite to be crystallized from melt makes it promising phase for actinides, rare earths and some other fission and corrosion products at using a melting route to nuclear waste forms including cold crucible induction melting and self-propagating high-temperature synthesis.

Highly disordered ionic distribution in α‐dicalcium silicate for structure relaxation

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Dicalcium silicate (Ca 2 SiO 4 , C2S) is a typical compound that appears in different fields related to ceramic materials such as building (e.g., cements and clinker), 1-5 biomaterials, [6][7][8] nuclear waste depository, [9][10][11] and metallurgical slag in metal production, [12][13][14][15] and plays significant roles due to its unique characteristic in accepting many kinds of doping elements. Among different kinds of C2S polymorphs (α: hexagonal, α': orthorhombic, β: monoclinic, γ: orthorhombic), most of the above fields focus on the β or γ polymorphs, which are metastable or stable phase at lower temperatures than 1123 K. Although various kinds of doping elements have been investigated for these polymorphs, the solid solubility of these elements are very limited.

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“…29,30 The physics of resin under pressure is similar to the seepage pr lar approaches have been used also to mod into a mold containing fiber mats. 23,[31][32][33][34][35][36][37][38] T in these cases to determine the resin flow ing times corresponding to different infilt Preceramic polycarbosilane polymers as precursors to make ceramic fibers 10,3 trix. [7][8][9]40,41 The chemical and volume ch pany ceramization of the polymer during documented in several studies.…”

“…Preceramic polycarbosilane polymers are commonly used as precursors to make ceramic fibers 10,39 and ceramic matrix. [7][8][9]40,41 The chemical and volume changes that accompany ceramization of the polymer during pyrolysis have been documented in several studies.…”