Temperature And High Pressure Synthesis And Property Studies On Manganese Borides

Transition metals(TMs)usually possess high bulk modulus(B)because high concentrations of valence electrons result in great repulsive forces within the crystal when it was compressed.However,due to the soft metallic bonds,TMs are facile to be sheared and have low hardness.Light elements(LEs)can form strong covalent bonds which limit the motion of dislocations and thereby increase the shear modulus.Transition metal light elements compounds possess the features both of transition metals and light elements and form a crystal structure of “reinforced concrete structure”.The abundant of valence electrons of transition metals will definitely give rise to diverse of physical property.Consequently,transition metal light element compounds are potential multi-functional hard materials.Magnetic materials have widely application in the area of electromechanical system and spintronics.Magnetic materials with high hardness possess the merits of chemical inertness,scratch resistance because of their strong bonds.The design and synthesis of magnetic materials with hardness is of great significance in the area of physics and materials.Within this context,we conduct systematic synthesis of manganese borides with the method of high pressure and high temperature.The crystal structure,mechanical property,magnetic property and bond feature of manganese borides were investigated by the combination of first principle calculations and experiments.The conclusions are listed below:We conduct high pressure and high temperature(HPHT)synthesis and first principle calculations of manganese borides.Mn2B(SG:I41/mcm),Cr B-type Mn B(SG:I41/amd),Fe B-type Mn B(SG:Pnma),Ta3B4-type Mn3B4(SG:Immm),Al B2-type Mn B2(SG:P6/mmm)and Mn B4(SG:P21/c)were synthesized by high pressure and high temperature method(HPHT).The Mn-B phase diagram under high pressure and high temperature was obtained based on the synthesis results.High pressure and high temperature synthesis of manganese borides has the following characteristics.First,more amorphous boron are needed to synthesis manganese borides when the boron content is higher than 50%.Second,the synthesis condition of Mn B4 was narrowed under HPHT.A new Cr B–type Mn B was fabricated under high pressure.The emergence of new crystal structure denotes that high pressure and high temperature method is an efficient way to enrich the crystal structure.Boron atoms form zigzag boron chains and incorporated into manganese metal matrix in the crystal structure of Mn B.Due to the increase of distance between manganese atoms expanded by inserting of boron backbone,there is a weak magnetic structure to strong ferromagnetic structure transition.Fe B-type Mn B possesses high Curie temperature,low coercive field and high saturated magnetization.The magnetic measurements results denote that Mn B is outstanding soft ferromagnetic material.Because of the strong covalent feature of zigzag boron chains,the hardness of Mn B is almost 16 GPa which is far higher than traditional ferromagnetic materials.The combination of high hardness and fantastic ferromagnetic properties in Fe B-type Mn B makes the goal,stronger,lighter and more energy efficient magnetic materials,closer.The strong mechanical properties and chemical inertness pave the way for Mn B in harsh condition applications.Graphene-like honeycomb boron framework emerged when the chemical ratio of manganese and boron arrived 1:2.Due to the empty π valence band of honeycomb borophenes,honeycomb borophenes is still out of our sight.However,graphene-like honeycomb boron sheets formed in Mn B2.X-ray photoelectron spectrum(XPS)combined with first principle calculation indicate that there is some electrons transformed to boron framework and filled it empty π valence band.Consequently,the graphene-like honeycomb boron framework was stabilized.The honeycomb boron layers were inserted into the manganese layers and it expands the distances between two manganese layers.Therefore,there is an anti-ferromagnetic to weak ferromagnetic transition because of graphene-like boron framework inserted into manganese host matrix.It is worthy to be noted that there is a strong covalent bonds in boron sheets which denote honeycomb boron sheets are of outstanding mechanical properties.The high hardness of Mn B2 in c-planes and ferromagnetic property can be utilized in the area of protection,magnetic devices used in harsh condition.Boron contents in manganese borides drive boron frameworks to derive regularly.There is isolated boron in Mn2 B,zigzag boron chains in Fe B-type Mn B,double zigzag boron chains in Mn3B4,graphene-like boron sheet in Mn B2 and three-dimensional boron cages in Mn B4.With the increasing of boron content,the hardness of manganese borides increase at first,then decrease,and followed by increasing.But at the boron content of Mn3B4 and Mn B2,there is a decrease of hardness which denotes that boron content is not linear with hardness.In summary,high valence electron contents,three-dimensional boron sub-structure is beneficial for higher hardness.However,laminar crystal structure is detrimental to higher hardness.With the increase of boron content,the magnetic properties of manganese borides undergo paramagnetic,ferromagnetic,anti-ferromagnetic and paramagnetic transition,respectively.Its derivation is closely related with the distance between manganese atoms and the exchange correlation integration which is tailored by the introducing of boron sub-structure frameworks into manganese atoms matrix.Based on influence of boron content on hardness and magnetic property,the proper transition metal atoms and proper light elements contents is vital for designing ferromagnetic hard materials.Proper transition metal atoms and light element content will endow multi-functional hard material.Furthermore,the proper crystal structure also should be taken into consideration to realize hard and multi-functional materials.In summary,the manganese borides were systematically synthesized by high pressure and high temperature method.The accurate Mn-B phase diagram under high pressure was obtained based on the synthesis condition of different manganese borides.There are multi-functional ferromagnetic hard materials,such as Cr B-type Mn B and Fe B-type Mn B,Al B2-type Mn B2 and Peierls distortion materials Mn B4.Under the conditions of high pressure and high temperature,the valence electron density,electron configuration,bond characteristics and crystal structures were tailored.A variety of crystal structure and boron sub-structure were realized in manganese borides which bring about fantastic physical property.The derivation of hardness and magnetic behavior of manganese borides was investigated with the increasing of boron content.The boron structure and physical property can be tuned by pressure,temperature and chemical ratio.The change denotes that hardness is not proportional to boron content.Furthermore,the magnetic behavior was changed with the different distance of manganese atoms.This work points out the principle for designing ferromagnetic materials with outstanding mechanical property.