Introduction

For the last few decades, double diphosphates of a general formula M ILnP2O7 (M I = Li, Na, K, Rb, Cs, Tl, Ag; M III = Al, Ga, In, Fe, Sc, Y, Ln, transition metal) have attracted considerable interest [1–19]. It has been primarily focused on the crystal structure, synthesis methods and prospective practical applications. These compounds are mainly obtained in the solidstate reaction by annealing various mixtures at different temperature and time range. As starting reaction mixtures the following compositions were used:

  1. 1.

    Ln 2O3 and M IPO3 using a large amount of an alkali metaphosphate, the excess of which was washed away with hot water, sometimes with lithium chloride [1–4]; the annealing was carried out in the temperature range 550–750 °C.

  2. 2.

    Stoichiometric amounts of substrate phosphates LnPO4 and M IPO3; the synthesis was carried out in the temperature range 400–900 °C [5–8].

  3. 3.

    Carbonates or nitrates of alkali metals M I2 CO3/M INO3, Ln 2O3 and (NH4)2HPO4; the synthesis proceeded in the temperature range 400–1200 °C [9–18].

A review of the literature shows that double diphosphates in the pure phase form as well as doped with various ions (mainly Ln) have interesting optical and dielectric properties, which indicates the nature of the applications as scintillators or phosphor materials based on A I M IIIP2O7-type diphosphates [16–23] or as ionic conductors [2, 4, 11]. These compounds have diverse crystal structures. They crystallize in the following systems: a monoclinic (lowtemperature form, various SG), an orthorhombic (highertemperature form; various SG), a hexagonal (the highest temperature form for Y, Tb, Dy and Er, but crystallographic data have not been given [1, 9]) and a tetragonal (pseudo-pyrophosphate-type PPP) [15]. It has been found that the formation of MIMIIIP2O7 double diphosphates and their crystal structures depend on the ionic radii ratio r IM /r IIIM [9–11]. According to [1] the thermal stability of the compounds, where M I = K, Rb, Cs and M III = Tb–Lu, increases with the alkali metal atomic number (i.e. in the series K–Rb–Cs), and as the lanthanide atomic number increases. They melt incongruently forming LnPO4 [1].

Double diphosphates, according to their stoichiometric composition, should be present in systems of the LnPO4–M IPO3 type, in the molar ratio of the starting compounds equal 1:1. Determination of the phase relationships in such systems, particularly in the entire range of temperature and composition, is relevant because it leads to discovery of new phases and also allows to determine their physicochemical properties. Our research team has developed phase diagrams of the following systems: LnPO4–KPO3 (where: Ln = La, Ce, Nd, Y), LnPO4–NaPO3 (where: Ln = Ce, Nd, Y) [5–9, 24–26]. It turned out that for M = K, double diphosphate of formula KYP2O7, only occurs in the YPO4–KPO3 system. It melts incongruently at 1000 °C according to the reaction: KYP2O7 → YPO4 + liquid. In the other LnPO4–KPO3 systems the compounds are not formed; these are simple systems of an eutectic type hereas in LnPO4–NaPO3 systems double diphosphates are formed only for Ln = Ce and Nd. These are NaCeP2O7 and NaNdP2O7. In the YPO4–NaPO3 system Na2YP3O10 tripolyphosphate is formed; it corresponds to the molar ratio of the starting components YPO4:NaPO3 = 1:2. Three mentioned above compounds melt incongruently at 800 (Ce), 790 (Nd) and 640 °C (Y), respectively, giving the corresponding lanthanide orthophosphate LnPO4 and liquid.

This paper presents the results of investigations of the phase equilibria in the ErPO4–KPO3 system. Its phase diagram is not known. In the literature, there are numerous reports on the starting phosphates, i.e. ErPO4 and KPO3. It is known that both compounds melt congruently at 1896 ± 20 °C [27, 28] and 810 ± 20 °C [29], respectively. Their polymorphism has been investigated by plenty of authors (see, e.g. [28–33]). Erbium orthophosphate ErPO4, is related to the LnPO4 group with xenotime structure, isostructural to zircon (ZrSiO4) and it crystallizes in the tetragonal system [SG I41/amd, a = 6.8614(5), c = 6.0082(9) Å, Z = 4] [33]. ErPO4 exists in one polymorphic form [30].

Potassium metaphosphate KPO3 melts congruently at 810 °C and, as the other alkali metal metaphosphates, tends to solidify not into a crystalline but into a glass form. It undergoes two polymorphic transformations: α/β at 449 °C and β/γ at 665 °C [29]. Decomposition of the compound begins only in the liquid phase at 1160 °C [33] (i.e. substantially above the melting point).

It is known from the literature that KErP2O7 exists. According to [9], the compound crystallizes in the orthorhombic system [a = 9.198(8), b = 12.209(8), c = 5.698(5) Å] hereas from [1], this diphosphate occurs in three polymorphic modifications: the low-temperature one (type I) crystallizes in the monoclinic system [a = 7.5670(3), b = 10.9090(3), c = 8.5830(3) Å and β = 106.84(3)°] PDF 00-049-0766, the orthorhombic system (type III) [a = 9.1950(3), b = 12.2160(5), c = 5.7040(1) Å] PDF 00-049-0767, and the hightemperature hexagonal system (type IV) [a = 9.5530(3), and c = 12.6380(4) Å] PDF 00-049-0768. The polymorphic transformation from the monoclinic to the orthorhombic form in KErP2O7 takes place in the temperature range 350–370 °C [1], but the temperature of the next polymorphic transformation (the orthorhombic to the hexagonal) is not known. The KErP2O7(III) ↔ KErP2O7(IV) transformation is reversible. However, rapid cooling of KErP2O7(IV) from high temperature do not completely convert it into KErP2O7(III) [1].

Experimental

Materials and samples preparation

The following commercial materials: Er2O3 (99.9 %, Aldrich), and NH4H2PO4, (NH4)2HPO4, KNO3, K2CO3, KH2PO4 (POCh)—all analytically pure were used to prepare the test samples from the ErPO4–K3PO4 system. The erbium orthophosphate ErPO4 was synthesized from Er2O3 and NH4H2PO4 by the method described in [34]. Potassium phosphate KPO3 was obtained KH2PO4 by dehydration at 350 °C for 2 h.

Samples for investigations were presynthesized by the reaction in the solid phase. The substrates were weighed out in fixed amounts, thoroughly mixed, rubbed in an agate mortar, and then sintered. The sintering temperature and time were determined experimentally.

Physicochemical characterization

Phase equilibria in the ErPO4–KPO3 system were investigated by thermoanalytical methods DTA/TG/DSC, X-ray powder diffraction XRD and FTIR spectroscopy.

The DSC/TG analysis during heating carried out by using a calorimeter SETSYS™ (TG-DSC 1500; SETARAM) up to 1300 °C (heating rate: 10 K min−1, argon atmosphere, platinum crucibles; mass of samples 15–30 mg). The DTA/TGheating was performed by means of a derivatograph type 3427 (MOM, Hungary) within temperature range of 20–1400 °C with a heating rate 5 °C min−1. Platinum crucibles and an air atmosphere were used; mass of samples was 400–600 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 were also conducted in a horizontal resistance furnace with molybdenum winding, under argon. Presynthesized samples of about 3 g mass were pressed into pellets and placed in boats made of PtRh30 alloy. Temperature points at which the samples began to melt and disappeared from the visual field were read with an optical pyrometer. For the samples that melt in a range of temperature, the points determined are approximate. The optical pyrometer was calibrated against the melting points of Ca2P2O7 (1353 °C), Na3PO4 (1583 °C) and Ca3(PO4)2 (1810 °C).

The X-ray powder diffraction (XRD) was used to determine the phase composition of final powders. The phase analysis was made using D5000 diffractometer (Siemens, Germany). The CuKα radiation (λ = 0.154 nm) was applied in measurements at room temperature in an angle range of 2θ: 10°–60° with 2θ scanning speed 0.04° s−1 and count-time of 2 s/step.

The high temperature (up to 950 °C) of X-ray diffraction (XRD) method with DRON-2 diffractometer using Fefiltered CoKα X-ray beam (λ = 0.179 nm) was taken for identification the crystal phases in examined samples. The diffraction was taken in 2θ range of 15°–45° and in a scan mode of step recording every Δ2θ = 0.05°. The crystal structure of the samples was identified using XRayan program by comparing the interplanar crystal spacing d and intensities of reflections I of the investigate samples with powder diffraction files (PDF) data. The quantity analysis and the crystallite sizes were calculated using RTG-3 program.

The infrared spectra at room temperature were measured in the spectral range 400–4000 cm−1 using a FTIR Biorad 575C Spectrometer. The samples were prepared in KBr pellet suspension. The spectral resolution was 2 cm−1.

Results and discussion

Phase equilibria occurring in the ErPO4–KPO3 system have been investigated over the entire range of composition, up to 1800 °C, by DTA/TG/DSC thermoanalytical methods, powder X-ray diffraction (XRD), and FTIR spectroscopy.

The experimental work started with obtaining KErP2O7 diphosphate in the pure phase form. For this purpose, various substrates and various synthesis conditions were applied (Table 1). Attempts to obtain pure phase KErP2O7 in the reaction of ErPO4 with KPO3 (Table 1, sample S1) failed. The starting phosphates were not reactive at lower temperature, the samples were always a mixture of ErPO4 and KPO3, and at 800 °C—a mixture of KErP2O7 and ErPO4.

Table 1 Preparation of KErP2O7
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The methods used for samples S2–S6 (Table 1) yielded pure phase KErP2O7. However, its crystallographic structure depended on the applied temperature during heat treatment. In Fig. 1 XRD patterns of the samples S2–S6 (Table 1) at room temperature (SIEMENS D5000) are presented. In the diffraction patterns of the samples S2, S3 and S5 reflections characteristic of the β-KErP2O7 monoclinic structure (PDF 00-049-0766) are visible, whereas XRD patterns of the samples S4 and S6 are characteristic of the orthorhombic α-KErP2O7 structure (PDF 00-049-0767).

Fig. 1
figure 1

The XRD patterns of KErP2O7 at room temperature

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Figure 2 presents infrared spectra of the products obtained by solidstate reaction (samples S2–S4) and Pechini method (S5 and S6). FTIR spectroscopic studies in the range 400–1400 cm−1 confirmed the phase purity of the obtained KErP2O7. It is known from the literature reports concerning spectroscopic research on double diphosphates of A I M IIIP2O7 type that the bands characteristic for (P2O7)4− group are present in the wavenumber ranges: 1080–1250, 975–1070, 910–950, 720–750 and 370–680 cm−1 and they are assigned respectively to the vibrations: ν as(PO3), ν s(PO3), ν as(POP), ν s(POP) and ν(PO3) [12, 13, 20]. In the spectra of KErP2O7 (S2–S6) the range of the bands occurrence is the same as in the works [12, 13, 20], which confirms purity of the obtained products. Slight shifts of the POP bridges stretching vibration bands in the β-phase spectrum (S2, S3, and S5) in relation to that of the α-form (S4 and S6) are due to the structural differences between the hightemperature orthorhombic and the low-temperature monoclinic phase. A similar effect was observed and described in the works [13, 20].

Fig. 2
figure 2

FTIR spectra of KErP2O7

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Obtaining β-KErP2O7 in the samples S3 and S5 was quite surprising, since in view of the work [1] the transition from the monoclinic to the orthorhombic modification occurs in the temperature range 350–370 °C and at the applied synthesis conditions α-KErP2O7 should be obtained. According to [1] discussed KErP2O7 diphosphate still has the highest temperature form with a hexagonal structure. Therefore, we found it necessary to conduct additional hightemperature XRD studies (DRON-2). The measurements were on the sample S2 (Table 1), which in the XRD studies carried out at room temperature had the β structure.

Figure 3a presents the XRD patterns of the sample S2 annealed for 1 h sequentially at the temperature 300, 350, 400, 750, 800 and 950 °C. Reflections characteristic of the monoclinic potassium erbium double diphosphate (KErP2O7) structure were observed at the temperature 300, 350 and 400 °C. Diffraction patterns of the sample annealed at 750 and 800 °C consisted of reflections indicating the presence of a mixture of the β and α modifications. At 950 °C, in addition to the α phase reflections, there were also reflections from erbium orthophosphate ErPO4. The obtained results indicated that short annealing time (1 h) in the temperature range 300–800 °C did not result in the formation of the pure phase α modification and annealing of KErP2O7 at 950 °C leads to its decomposition (reflections from ErPO4). KErP2O7 is unstable at this temperature, so its hexagonal modification cannot exist.

Fig. 3
figure 3

a The high-temperature XRD patterns of sample S2. b The XRD patterns of the sample S3 annealed additionally at 750 °C in various times. c The XRD patterns of the sample S4 annealed at 250 °C in various times

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Based on the discussed above XRD studies the maximum temperature of the hightemperature synthesis of α modification was determined. It is 750 °C (Table 1). In order to avoid any decomposition of KErP2O7 during prolonged heating the sintering temperature 750 °C was chosen. Also, based on the XRD studies the synthesis time was evaluated. Figure 3b shows the diffraction patterns of the sample S3 additionally annealed at 750 °C for 24, 48, and 72 h. It appeared that the transformation β to α modification occurs only after long annealing time (72 h). Also, by XRD analysis we examined the kinetics of the transformation from α to β modification. Figure 3c shows the diffraction patterns of the sample with the α structure (sample S4) annealed at 250 °C for 1 and 2 weeks. We found that in both cases a mixture of the two KErP2O7 modifications (the monoclinic and orthorhombic) were present. In summary, the results of X-ray studies indicate that:

  • KErP2O7 phosphate occurs in two polymorphic forms: β-monoclinic and α-orthorhombic.

  • KErP2O7 polymorphic transformation is enantiotropic but the transformation of one modification into another is very difficult. Due to the kinetic limitations and a slower diffusion of reagents in the solid phase and a significant difference in crystal structures of both modifications it requires the use of an appropriate heat treatment, hexagonal modification of KErP2O7 does not exist.

In order to determine the thermal characteristics of KErP2O7 phosphate, i.e. to establish its thermal stability, melting point (or decomposition temperature), and verification of the information on the existence of a hightemperature hexagonal polymorph DTA/DSC/TG thermal studies was carried out. The samples synthesized by different procedures shown in Table 1 were used in the studies. Figure 4 shows the DSC-heating curves of the samples S2–S6. On the DSC-heating curves of the S2, S3 and S5 samples only one endothermic effect with corresponding temperature 1025, 1012, 1006 °C, respectively, was observed. An analogous endothermic effect, but at slightly higher temperature, occurred on the DSC-heating curves of the samples S4 (1021 °C) and S6 (1029 °C). Another very weak endothermic effect was observed for these samples at approx. 358 °C. It can be attributed to β/α-KErP2O7 polymorphic transformation. The occurrence of this effect in the samples S4 and S6, and its absence in the samples S2, S3 and S5 confirms the results of the mentioned above XRD studies.

Fig. 4
figure 4

The DSC heating curves of KErP2O7

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Due to the complicated conditions of the pure phase KErP2O7 phosphate synthesis, to establish phase equilibria and to construct the ErPO4–KPO3 phase diagram two series of samples obtained from:

  • ErPO4 and KErP2O7 various mole ratio mixtures (sample S4—the orthorhombic modification), to determine the phase equilibria in the composition range 0–31.05 mass% KPO3.

  • KErP2O7 (sample S4—the orthorhombic modification) and KPO3 various mole ratio mixtures, to determine the phase equilibria in the rest of this system, i.e. richer in KPO3 (>31.05 mass% KPO3), were examined.

The prepared mixtures were sintered at 750 °C for 24 h (platinum crucibles). The resulting sinters were slowly cooled down to room temperature, then crushed and ground thoroughly. Their phase composition was monitored by X-ray diffraction analysis.

Figure 5 shows the proposed phase diagram of the ErPO4–KPO3 system developed on the basis of the results of thermal studies. The endothermic effects seen on the DSC-heating curves of the samples at approx. 1025 °C (in the composition range from 26.4 to 75 mass% KPO3) are assigned to peritectic KErP2O7 decomposition that occurs according to the reaction KErP2O7(s) → ErPO4(s) + L. The β/α-KErP2O7 polymorphic transformation is reflected on the DSC-heating curves as a very weak effect at approx. 358 °C. They are only found in the samples with compositions included in the concentration range 21.7–72.4 mass% KPO3. Figure 6 shows the DTA/TG-heating curves of two selected samples from the composition range. According to Fig. 6 the mass loss observed on the TG curve starts at app. 1120 °C, so above the temperature of incongruent decomposition of KErP2O7.

Fig. 5
figure 5

The phase diagram of ErPO4–KPO3 system. (Bullet symbols onset temperatures of endothermic effects appeared on DTA/DSC curves, times symbols melting temperatures of samples read by an optical pyrometer)

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Fig. 6
figure 6

DTA/TG curves of heating of two selected samples from the ErPO4–KPO3 system

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In the ErPO4–KPO3 system the eutectic of composition: 92 mass% KPO3, 18 mass% ErPO4, exists, the melting point of which is 798 °C.

Potassium metaphosphate KPO3 undergoes two polymorphic transformations: α/β at 449 °C and β/γ at 665 °C [29]. They were reflected on the DSC curves of the samples as thermal effects in the composition range 41.4–100 mass% KPO3. According to [33], KPO3 metaphosphate decomposes at 1160 °C, so at the temperature considerably higher than the melting point. The mass loss accompanying the process does not affect the results of the studies on the phase equilibria in the ErPO4–KPO3 system.

Conclusions

The phase diagram of the ErPO4–KPO3 system was constructed on the basis of the comprehensive experimental investigations. One intermediate compound with the formula KErP2O7 occurs in this system. It decomposes peritectically at about 1025 °C and exists in two polymorphic forms: monoclinic and orthorhombic.