Phase equilibria in the ErPO4–K3PO4 system
D. Piotrowska
0
T. Znamierowska
0
I. Szczygie
0
0
D. Piotrowska (&) T. Znamierowska I. Szczygie Department of Inorganic Chemistry, Faculty of Engineering and Economics, Wrocaw University of Economics
, Komandorska 118/120, 53-345 Wrocaw,
Poland
The phase equilibria occurring in the ErPO4K3PO4 system were investigated by the thermal analysis, FTIR, and X-ray powder diffraction methods. On the basis of obtained results, the related phase diagram is proposed. This system includes one intermediate compound, K3Er(PO4)2; the double phosphate melts incongruently at 1355 C and occurs in two polymorphic forms; transformation b/a-K3Er(PO4)2 proceeds at 420 C. The eutectic occurs at the composition of 58.5 wt% K3PO4, 41.5 wt% ErPO4 at 1317 C.
-
Many papers about double phosphates of the general formula
MI3Ln(PO4)2 (where MI denotes an alkali metal and Ln is a
rare earth element or Y or Sc) have been published. The
information is mainly focused on the preparation methods,
crystalline structure, and application possibilities of those
compounds. According to the data, lanthanide-alkali metal
double phosphates are of technological importance for
applications in optics and electronics [110].
In view of relevant information from the literature, double
phosphates of the formula MI3Ln(PO4)2 should occur in the
systems of LnPO4MI3PO4 (where Ln denotes a rare earth
element or yttrium, and MI does an alkali metal). According
to our research group results, such compounds occur in the
Ln2O3MI2OP2O5 oxide systems on the LnPO4MI3PO4
subsystems, where Ln = La, Ce, Nd, Y and MI = Na, K, Rb
[1118]. It should be noted that, in the system YPO4
Na3PO4, two intermediate compounds of Na3Y(PO4)2 and
Na3Y2(PO4)3 occur; both compounds melt congruently.
Also, in the system YPO4Rb3PO4, two intermediate
compounds occur; namely Rb3Y(PO4)2 which melts
congruently, and the Rb3Y2(PO4)3 which is unstable and
decomposes in the temperature range between 1300 and
1330 C. In each of the other investigated systems, a single
I
intermediate of M3Ln(PO4)2 occurs; it melts incongruently.
I
Double phosphates M3Ln(PO4)2 are usually obtained in a
solid-state reactions by sintering an equimolar mixture of
MI3PO4 and LnPO4.
In the present paper, the results of investigation of the
ErPO4K3PO4 subsystem are presented. The related phase
diagram has not been reported so far. It is known from the
literature that K3Er(PO4)2 exists as well. According to
Refs. [19, 20], the compound appears in two polymorphic
modifications. The high-temperature one crystallizes in
the hexagonal system (S.G. P 3, glaserite-type) and the
low-temperature one does in the monoclinic system (S.G.
P21/m, a = 7.371(1), b = 5.595(1), c = 9.318(1) A , and
b = 90.90(1) ). A polymorphic transition in K3Er(PO4)2
exhibits at 436.4 C [19].
The parent orthophosphates ErPO4 and K3PO4 are
known for congruent melting at 1896 20 C [2123] and
1620 20 C [24], respectively. Polymorphism of both
orthophosphates was investigated by many authors (see,
e.g., [21, 2431]). Erbium orthophosphate, ErPO4, is
related to REPO4 group with xenotime structure, isostructural
to zircon (ZrSiO4). Orthophosphate ErPO4 crystallizes in
the tetragonal system (S.G. I41/amd, a = 6.8614(5), c =
6.0082(9) A , Z = 4) [21]. The compound exists in one
polymorphic form. According to the literature on K3PO4,
polymorphism has revealed significant disagreements. This
problem will be described in the Results section.
The following commercial materials: Er2O3 (Aldrich), and
NH4H2PO4, (NH4)2HPO4, K2CO3, K3PO4 3H2O (POCh)
all analytically pure were used to prepare the test samples
from the ErPO4K3PO4 system. The erbium
orthophosphate ErPO4 was synthesized from Er2O3 and NH4H2PO4
by the method described in [32]. Potassium orthophosphate
K3PO4 was obtained from K3PO4 3H2O by dehydration at
900 C for 1 h.
Phase equilibria in the ErPO4K3PO4 system were
investigated by thermoanalytical methods and X-ray
powder diffraction at room temperature.
Samples for investigations were presynthesized by the
reaction in the solid phase. The substrates were weighed out
in fixed amounts, thoroughly mixed (in weighing bottle),
rubbed in an agate mortar, and then sintered. The sintering
temperature and time were determined experimentally.
The DSC/TG analysis during heating was carried out
using a calorimeter SETSYSTM (TGDSC 1500;
SETARAM) up to 1300 C (heating rate: 10 K min-1, argon
atmosphere, platinum crucibles; mass of samples 1530 mg). The
DTA/TG-heating was performed by means of a
derivatograph type 3427 (MOM, Hungary) within temperature range
of 201400 C with a heating rate 5 C min-1. Platinum
crucibles and an air atmosphere were used; mass of samples
was 400600 mg. The standard substance was Al2O3. The
temperatures were measured by a Pt/PtRh10 thermocouple
standardized for the melting points of NaCl (801 C), K2SO4
(1070 C), Ca2P2O7 (1353 C), and the transition points of
K2SO4 (583 C). The high-temperature experiments (above
1400 C) were conducted (...truncated)