UTRIP 2011 Final Paper

Rachel Norman University of Strathclyde Glasgow

1.Prussian blue analogues (PBAs)

Introduction Many years ago PBAs were used as pigments in paintings, but now PBAs have interesting physical properties. The first is how humidity can change magnetic properties: for example, in CoII[CrIII(CN)6]2/3·zH2O

The sample colour changes from pink to purple, this is due to humidity dependence on the sample weight, i.e. as humidity is increased weight is increased. This is due to water molecules being adsorbed. As humidity is increased the magnetization is increased also, this is due to the geometrical change in the cobalt sites.

The next is high proton conductivity: for example, in CoII[CrIII(CN)6]2/3·5H2O (same complex as before)

Proton conductivity is the movement of protons through water molecules in a complex. As the temperature is increased the conductivity increases therefore the compound is considered as a proton conductor. The conductivity at 100% RH, 20°C is 1.2x10-3 şcm-1, which shows the product is super ion conductor. The activation energy can be determined by the ln(σT) vs T-1 plots, which is 0.22eV.

And lastly photomagnetism: for example, in RbMn[Fe(CN)6].

Light irradiation causes an electron to transfer from metal site to the other, this changes the spin state of the system and therefore the magnetism. When different light irradiation is applied again reverse electron transfer takes place. The system can switch between ferromagnetic and anti-ferromagnetic.

Synthesis

The basic reaction was as follows:

The synthesise of the material involved is shown below:

Results

RbNi-Fe

The composition of the compound was firstly determined. IR spectrum and XRD patterns will be described in more detail later. UV-Vis has a main peak at around 300 nm which is the [FeIII(CN)6]peak. There is also a secondary peak at 410 nm which is from the transition 3A2g 3T1g of the nickel.

CsNi-Fe

In the UV-Vis we can see a main peak at 300nm this is again the [FeIII(CN)6]peak. Another peak at 415nm is also observed to be 3A2g 3T1g of the nickel.

RbCo-Fe

Unfortunately the composition of this sample could not be determined as the Fe percentages were not compatible. The UV-Vis for this sample is different. There is a strong broad peak at 570nm. This is due to metal-metal charge transfer in mixed valence compounds i.e. RbCoIIIFeII RbCoIIFeIII. Explanation for the peak shift in IR spectra

CsNiFe has the highest frequency followed by RbNiFe and finally RbCoFe. This order is explained by π back donation. Usually a nitrogen atom donates its electrons to the metal but

in this case the metal also back donates its electrons to the nitrogen. As this occurs the metal-Nitrogen bond is made therefore weakening the nitrogen carbon bond causing a peak shift. The higher the electron negativity of the metal then the better it is at back donating.

Explanation of the x-ray diffraction pattern

The X-ray diffraction pattern tells us the structural information i.e .the space group and the lattice constants. The lattice constant is a distance between unit cells in the lattice from one metal ion to another in a molecule. All the samples above are cubic structures of similar size and we can therefore say they are PBAs. (~10Å)

Magnetic Property

The TC temperature decreases with a larger radius therefore we can say Cs has a larger radius complex than Rb. RbCo-Fe had no ferromagnetism due to a change of valence which would result in generating diamagnetic cobalt(II) centres.

Both RbNi-Fe and CsNi-Fe complexes show a relatively large coercive field of 4000Oe and 3500Oe respectively.

Impendance Analysis for RbCoFe

Result of fitting curve shows that this complex is a relatively high proton conducting material at ~10-4 S cm -1 at 100% RH, suggesting these compounds to be a good candidate for proton-conductive materials.

• Conclusion

To conclude, I successfully synthesized 3 kinds of alkali-cation-doped Prussian blue analogues. 2 of them were found out to be a ferromagnet with a relatively large coercive field (Hc ~ 4000 Oe). 1 of them showed high proton conductivity of about 10-4 S cm-1, suggesting these compounds to be a good candidate for proton-conductive materials.

2.Magnetic Properties Of ε-Fe2O3 In Relation To Particle Size

Iron Oxide has many phases. An epsilon phase exists between alpha and gamma phases. ε-Fe2O3 is an orthorhombic nano crystal which exists in a very narrow particle size range and is therefore hard to obtain. For technological advances the magnetic properties of iron oxide are desirable. Coercive field indicates the resistance of a ferromagnetic material becoming demagnetised. Ohkoshi lab has discovered this pure epsilon phase and the world’s largest coercive field of 20 kOe among metal oxides. My objective was to determine how the magnetic properties of ε-Fe2O3 changes with changing particle size. As smaller particles are of bigger interest to us as they can be used in high density magnetic memory devices.

Introduction

Synthesis of The synthesis involves the reverse micelle and sol-gel methods. Firstly two reverse micelle solutions were made one with iron ions and the other with ammonia. Octane and CTAB were added to both to make a reverse micelle. Solution 2 was introduced to solution 1 to form a precursor micelle. The TEOS was then added iron oxide micelle which formed a glass coat around the micelle. The material was then separated and sintered.

ε-Fe2O3

We carried out 2 measurements: X-ray diffraction (XRD) and magnetisation vs external field (MH) measurement.

Properties

Figure 1: XRD pattern sample A Figure 2: Coercive field curve sample A

XRD pattern shows that we have glass present which we would expect. Some α-Fe2O3 is present which impurity but mainly ε- Fe2O3 is present. The coercive field value is lower than the expected 20 kOe, this is probably due to the alpha impurity. . Generally when a substance is placed in an external field the spins align parallel to the field. When the field is removed the spins are free to move randomly. However in epsilon when the field is removed the spins remain in the same direction as it is a stable configuration.

By changing the concentration of iron ions we can change the particle size: as the concentration is decreased the amount of glass coating the particle is more and hence less aggregation occurs. Also by decreasing the sintering temperature, the glass coating is less soft and this again prevents aggregation, both giving a decrease in particle size.

Changing conditions

I then made a matrix to change the concentration of Fe and the sintering temperature. For my report I will focus on sample 3.

40x103

35

30

25

20

15

10

Intensity

70605040302010

2θ [deg]

ε-Fe2O3

α-Fe2O3

SiO2

1 Sample 2 3

[Fe3+]/mol L-1 0.024 0.048 0.072

Sintering Temperature /oC

1000 1050 1075 1100

1000 1050 1075 1100

1000 1050 1075 1100

Results

Sample 3

XRD

1000°C 1050°C 1075°C 1100°C

If we look at the XRD graphs and zoom in, at 1100°C we can see there are alpha particles present. By sintering at a lower temperature the glass isn’t as soft and therefore particles can`t aggregate as well. Therefore you can see the peaks begin to broaden which suggest smaller particles are being formed every time the temperature is lowered. This can be described by Gibbs Free Energy Curve. Alpha particles are more stable at a larger diameter followed by epsilon at smaller diameter and then gamma. Therefore at 1000, 1050, 1075°C we can see there are no gamma or alpha phases present in XRD, only epsilon therefore we can assume a small diameter.

MH

As sintering temperature is decreased the coercive field decreases

By plotting the coercive field (HC) values we can furthermore say we have a single domain structure. By lowering the sintering temperature further we can obtain superparamagnetic ε-Fe2O3 particles

Conclusion

To conclude, we successfully synthesized ε-Fe2O3 of small particle size and observed a decrease in the HC value. No gamma phase was detected therefore we assume that superparamagnetic ε-Fe2O3 was obtained.

25

20

15

10

5

0

Coercive Field [kOe]

1300120011001000900800

Sintering Temperature [℃]

1.Prussian blue analogues (PBAs)2.Magnetic Properties Of ε-Fe2O3 In Relation To Particle Size