Facile synthesis of novel, known, and low-valent transition metal phosphates via reductive phosphatization

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Syntheses
The general synthetic procedure of TMPs via the molten salt method is illustrated in Figure S1.In a typical preparation, metal oxide powder (TiO 2 , V 2 O 5 , Cr 2 O 3, MnO 2 , Fe 2 O 3 ) is mixed with a surplus of ammonium hypophosphite (NH 4 H 2 PO 2 ) with weight ratios up to 1/10 and heated for 2 h in a tube furnace under argon flow.Finally, the sample is cooled down and washed with de-ionized water until pH 6 is achieved for removing excess phosphates from the crystalline material.

Figure S1
. Illustration of the general synthesis procedure for the preparation of TMPs by the molten salt method, starting from a solid mixture of ammonium hypophosphite and metal oxide.

Check of PH 3 as spectator species
Ammonium hypophosphite was decomposed at 500°C to release phosphane gas (PH 3 ).The formed PH 3 then was passed over TiO 2 (P25) in a continuous flow reactor under inert gas conditions at that temperature.Subsequently, the metal oxide powder was characterized via XRD which showed that no changes in crystallinity and phase composition of the precursor occurred.Thus, PH 3 does not react with the titanium oxide under these reaction conditions.

Synthesis of Ti(III)p at 300 °C
The synthesis of Ti(III)p was performed from a dry mixture of TiO 2 (P 25, Degussa, phase mixture of anatase and rutile, ≥ 99.5%) and NH 4 (H 2 PO 2 ) (Fluka, ≥ 97.0%) with a weight ratio of 1/10.The synthesis was tested for batches in a range between 1 g and 10 g without any technical complications or deviations of the product crystallinity and purity.The mixture was filled in a ceramic crucible and heated in a tube furnace at 300 °C for 2 h under Ar flow (100 mL/min).A heating ramp of 10 °C/min was used up to 250 °C which then was decreased to 2 °C/min up to 300 °C.Finally, the sample was cooled down and washed with de-ionized water until pH 6 was achieved in the effluent.The powdery product was dried in air at 80 °C.
The synthesis of Ti(III)p is accompanied by several temperature depending events which are illustrated in Figure S2.
Figure S2.Photos taken during the preparation of Ti(III)p within a tube furnace at increasing temperatures.
Up to 200 °C the precursor mixture keeps a powdery form before ammonium hypophosphite starts to melt at 215°C (Figure S2).Partial thermal decomposition of the ammonium hypophosphite into phosphane and ammonium phosphate starts at temperatures above 230°C.Above 245°C the hypophosphite starts to react with titanium oxide as indicated by a deep purple coloration of the melt which is characteristic for the formation of titanium(III) species.Finally, the melt solidifies after the whole ammonium hypophosphite has reacted or decomposed.

Safety note:
The thermal decomposition of ammonium hypophosphite causes the formation of gaseous phosphane (PH 3 , CAS: 7803-51-2) which is known as a strong respiratory poison.Therefore, the preparation of TMPs by the presented molten salt method has to be implemented exclusively in closed systems under continuous inert gas flow.
Figure S3 shows the XRD pattern of Ti(III)p.

Synthesis of Ti(IV)p at 500 °C
Ti(III)p obtained via the synthetic procedure described above was filled in a ceramic crucible and thermally treated at 500 °C under Ar flow (100 mL/min) for 4 h using a heating rate of 10 °C/min.Finally, the resulting white-yellowish powder was washed with de-ionized water and dried in air at 80 °C for 12 h.The phase transformation was tested for batches ranging from 100 mg to 1 g without any deviations of the product crystallinity and purity.

Synthesis of Ti(PO 3 ) 3 at 500 °C
The procedure used for the synthesis of Ti(PO 3 ) 3 is similar to that described for Ti(III)P with a difference in heating rate and temperature.A mixture of TiO 2 (P 25, Degussa) and NH 4 (H 2 PO 2 ) (Fluka, ≥ 97.0%) with a weight ratio of 1/10 was filled in a ceramic crucible and heated in a tube furnace at 500 °C for 2 h under Ar flow (100 mL/min) using a heating rate of 10 °C/min.Finally, the sample was cooled down and washed with de-ionized water until pH 6 was achieved.The powdery product was dried at 80 °C in air.The synthesis was tested for batches ranging from 1 g to 5 g without any technical complications or deviations of the product crystallinity and purity.Figure S9 shows the XRD pattern of Ti(PO 3 ) 3 and Figure S10 shows SEM images of Ti(PO 3 ) 3.

Synthesis of V(PO 3 ) 3 at 300 °C
The synthesis of V(PO 3 ) 3 was performed from a mixture of 0.2 g V 2 O 5 (Merck, ≥ 99%) and 2 g NH 4 (H 2 PO 2 ) (Fluka, ≥ 97.0%) .The mixture was filled in a ceramic crucible and heated in a tube furnace at 300 °C for 2 h under Ar flow (100 mL/min).A heating ramp of 10 °C/min was used up to 250 °C which then was decreased to 2 °C/min up to 300 °C.
Finally, the sample was cooled down and washed with de-ionized water until pH 6 was achieved.An additional washing step with ethanol was performed to avoid partial dissolution of the product by residual washing water during the drying process.The powdery product was dried in air at 80 °C.

Synthesis of V(PO 3 ) 3 at 500 °C
The procedure used for the synthesis of V(PO 3 ) 3 at 500°C is similar to those described above for 300 °C.The synthesis was performed from a mixture of 1 g V 2 O 5 (Merck, ≥ 99%) and 10 g NH 4 (H 2 PO 2 ) (Fluka, ≥ 97.0%).The mixture was filled in a ceramic crucible and heated in a tube furnace at 500 °C for 10 h under Ar flow (100 mL/min).A heating ramp of 10 °C/min was used up to 250 °C which then was decreased to 2 °C/min up to 500 °C Finally, the sample was cooled down and washed with de-ionized water until pH 6 was achieved in the effluent.An additional washing step with ethanol was carried out to avoid partial dissolution of the product by residual washing water during the drying process.The powdery product was dried in air at 80 °C. Figure S11 shows the XRD pattern of V(PO 3 ) 3 and Figure S12 shows SEM images of V(PO 3 ) 3.

Synthesis of Cr(NH 4 )HP 3 O 10 at 300 °C
The synthesis of Cr(NH 4 )HP 3 O 10 was performed from a mixture of 0.5 g Cr 2 O 3 (Merck, ≥ 98%) and 5 g NH 4 (H 2 PO 2 ) (Fluka, ≥ 97.0%).The mixture was filled in a ceramic crucible and heated in a tube furnace at 300 °C for 10 h under Ar flow (100 mL/min).A heating ramp of 10 °C/min was used up to 250 °C which then was decreased to 2 °C/min up to 300 °C.Finally, the sample was cooled down and washed with de-ionized water until pH 6 was achieved in the effluent.
An additional washing step with ethanol was carried out to avoid partial dissolution of the product by residual washing water during the drying process.The powdery product was dried at 80 °C in a ventilation oven over night and used for analysis.Figure S13 shows the XRD pattern of Cr(NH 4 )HP 3 O 10 .

Synthesis of Cr 2 (P 6 O 18 ) at 500 °C
The synthesis of Cr 2 (P 6 O 18 ) was performed from a mixture of 0.2 g Cr 2 O 3 (Merck, ≥ 98%) and 2 g NH 4 (H 2 PO 2 ) (Fluka, ≥ 97.0%).The mixture was filled in a ceramic crucible and heated in a tube furnace at 500 °C for 4 h under Ar flow (100 mL/min) with a heating ramp of of 5 °C/min.Finally, the sample was cooled down and washed with de-ionized water until pH 6 was achieved in the effluent.An additional washing step with ethanol was carried out to avoid partial dissolution of the product by residual washing water during the drying process.The powdery product was dried at 80 °C in a ventilation oven over night and used for analysis.Figure S14 shows the XRD pattern of Cr 2 (P 6 O 18 ).precursor.

Synthesis of Mn 2 (P 4 O 12 ) at 500 °C
The synthesis of Mn 2 (P 4 O 12 ) was performed from a mixture of 0.2 g MnO 2 (Merck, ≥ 99.0%) and 1 g NH 4 (H 2 PO 2 ) (Fluka, ≥ 97.0%).The mixture was filled in a ceramic crucible and heated in a tube furnace at 500 °C for 4 h under Ar flow (100 mL/min) with a heating ramp of of 10 °C/min.Finally, the sample was cooled down and washed with de-ionized water until pH 6 was achieved in the effluent.The powdery product was dried in air at 80 °C.

Synthesis of Fe(II)p at 300 °C
The synthesis of the novel Fe(II)p compound was performed from a mixture of 0.5 g Fe 2 O 3 (Riedel-de-Haen, ≥ 97%) and 5 g NH 4 (H 2 PO 2 ) (Fluka, ≥ 97%).The mixture was filled in a ceramic crucible and heated in a tube furnace at 300 °C for 10 h under Ar flow (100 mL/min).A heating ramp of 10 °C/min was used up to 250 °C which was then decreased to 2 °C/min up to 300 °C.Finally, the sample was cooled down and washed with de-ionized water until pH 6 was achieved in the effluent.The powdery product was dried in air at 80 °C over night.
Figure S16 shows the XRD pattern of the novel iron(II) phosphate phase (Fe(II)p).Figure S18 shows SEM images of Fe(II)p.

Synthesis of Fe 2 (P 4 O 12 ) at 500 °C
The synthesis of Fe 2 (P 4 O 12 ) was performed from a mixture of 0.2 g Fe 2 O 3 (Riedel-de-Haen, ≥ 97%) and 2 g NH 4 (H 2 PO 2 ) (Fluka, ≥ 97%).The mixture was filled in a ceramic crucible and heated in a tube furnace at 500 °C for 10 h under Ar flow (100 mL/min) with a heating ramp of of 10 °C/min.Finally, the sample was cooled down and washed with de-ionized water until pH 6 was achieved in the effluent.The powdery product was dried in air at 80 °C over night.Figure S19 shows the XRD pattern of Fe 2 (P 4 O 12 ).precursor.

Figure
S6 shows the XRD pattern of Ti(IV)p.

Figure S5 .
Figure S5.TG curve of Ti(III)p → Ti(IV)p phase transformation and associated mass signals of hydrogen (m/z 2), ammonia (m/z 17) and water (m/z 17, 18).The mass signals have been recorded from mass spectra of the exhaust gas of the TG/DSC instrument.

Figure S7 .
Figure S7. 31P MAS NMR spectra of Ti(IV)p prepared via phase transformation from the Ti(III)p precursor phase.The Ti(III)p samples were prepared at different batches of 1 g, 5 g and 10 g.

Figure
FigureS7shows 31 P MAS NMR spectra of three Ti(IV)p samples, which were prepared via phase transformation from the presented Ti(III)p precursor phase.The Ti(III)p precursor material was synthesized in different batches of 1 g, 5 g and 10 g (FigureS7 a-c).The signals around -30 ppm can be attributed to the pyrophosphate units of the crystalline Ti(IV)p samples.Spectra a) and b) show two small signals between 0 and -10 ppm which are characteristic for free ortho-and pyrophosphates which are not part of the crystal structure.[1]The 31 P MAS NMR spectra of Ti(IV)p show no additional phases and low amounts of amorphous parts, which indicates also a good purity of the Ti(III)p precursor material.Upscaling without losses in crystallinity and a high purity are features of the presented molten salt method.

Figure S8 .
Figure S8.SEM images of a-c) Ti(III)p and d-f) Ti(IV)p synthesized via reductive phosphatization.

Figure S12 .
Figure S12.SEM images at different magnification of V(PO 3 ) 3 synthesized via reductive phosphatization of V 2 O 3 in a melt of ammonium hypophosphite.

Figure
S15 shows the XRD pattern of Mn 2 (P 4 O 12 ).

Figure S16 .
Figure S16.XRD pattern of Fe(II)p synthesized by the conversion of Fe 2 O 3 in a melt of ammonium hypophosphite at 300 °C.Lines indicate the reflections of Fe 2 O 3 (▬) (PDF 00-056-1302) precursor.

Figure
Figure S17 shows the 57 Fe Mössbauer spectrum of Fe(II)p.Two different Mössbauer signals with isomer shifts in a range expected for Fe(II) high spin species are illustrated.[2]While the quadrupole splitting of first component (green line) is similar to that observed in LiFePO 4 , the smaller quadrupole splitting of the second component (blue line) is quite small for Fe(II) high spin species.The sharp resonance signals of the spectrum indicate that the sample contains no significant amorphous parts of iron phosphate.

Figure S17 .
Figure S17. 57Fe Mössbauer spectrum of Fe(II)p recorded at 80 K showing two Mössbauer sites with isomer shifts characteristic for Fe(II) species.

Figure S18 .
Figure S18.SEM images at different magnification of Fe(II)p synthesized by the conversion of Fe 2 O 3 in a melt of ammonium hypophosphite at 300 °C.